In EXACTING the same way as you "Add" any "Living" individua/person.
By Indicating/Selecting their 'Status' as "Living", rather than "Deceased.
The ONLY difference between, the "Living"; and, the "Deceased", is that ALL "Living" individuals/persons that you "Create" will be LOCATED in your "Private Spaces" in "Family Tree" of 'FamilySearch' - ONLY be 'seen'; and, 'accessible' to You.
We (Users/Patrons), CANNOT, 'see'; or, 'access', the "Living" individuals/persons that were "Create" by another User/Patrons - for reasons of "Privacy".
Here are some "Knowledge Articles" in 'FamilySearch':
How do I add living relatives to a person in Family Tree?
How do I add a family member to the pedigree in Family Tree?
How do I add a spouse or partner in Family Tree?
I hope this helps.
Just in case you were not aware ...
Most new (and, some old) Users/Patrons DO NOT understand the basic nature and premise of "Family Tree" of 'FamilySearch', when they join in.
Please let me explain ...
[ And, please bear with me; as, I am VERY 'Verbose' ... ]
We do not have our OWN "Tree" in "Family Tree" of 'FamilySearch'.
We ONLY have "Branches" (ie. Ancestral" lines), that are interconnected, in this SINGLE "One" World "Tree", for all of us, that is "Family Tree" of 'FamilySearch'.
"Family Tree" of 'FamilySearch' is NOT like 'On-Line' "Websites" (eg. "Ancestry_com"; or "MyHeritage_com"; or, the like); and/or, 'standalone' personal (computer) programmes (eg, the OLD, now no longer supported, "PAF"; or, "Ancestral Quest"; or, the like).
We DO NOT have "Private"/"Personal" 'Trees' in "Family Tree" of 'FamilySearch' like other 'On-Line' "Websites"; and/or, 'standalone' personal (computer) programmes.
We do not even, own; or, manage; and, are NOT even responsible for, the "Deceased" individuals/persons in "Family Tree" of 'FamilySearch'.
"Family Tree" of 'FamilySearch' is built on a "Open Edit" Platform - hence, why any registered User/Patron can "Edit" (ie. Add, Delete; and/or, Change) ANY "Deceased" individual/person in "Family Tree" of 'FamilySearch'.
You can certainly "Merge"/"Combine" any "Deceased" individual/person in "Family Tree" of 'FamilySearch' with another "Deceased" individual/person in "Family Tree" of 'FamilySearch', regardless of who the User/Patron was that initially created those "Deceased" individuals/persons; as, they DO NOT reside with the User/Patron who initially created them - they are "Public".
If fact, you can ONLY see "Deceased" individuals/persons in "Family Tree" of 'FamilySearch'; except, with this 'rider', that you can ALSO only see "Living" individual/person in "Family Tree" of 'FamilySearch' that YOU created; as, they reside in your "Private Spaces".
No other User/Patron can see ANY "Living" individuals/persons that were created by another User/Patron; as, they ONLY reside in the "Private Spaces" of the User/Patron who created them.
We CANNOT even see the ACTUAL Record in "Family Tree" of 'FamilySearch' of our IMMEDIATE Family (eg. Spouse; Children; Parents; Siblings; ETC; Etc; etc) if they are "Living"; and, they have their own 'FamilySearch' Account.
Of course, you can create your "Living" Family members; and, record them in "Family Tree" of 'FamilySearch'; but, they will ONLY be seen by you:
▬ You cannot "Share" them with any other User/Patron.
▬ You cannot "Merge"/'Combine" them with any other "Living" individual/person residing in the "Private Spaces" of another User//Patron.
There is CURRENTLY no mechanism in "Family Tree" of 'FamilySearch' for Users/Patrons, to "Share"; or, "give permission" to "Share", the "Living" individuals/persons in their own "Private Spaces" with/between other/another User(s)/Patron(s).
There are a myriad of "Privacy" Laws within the many various Countries and Unions throughout the World - it is a nightmare to negotiate.
"Family Tree" of 'FamilySearch' is used in many Countries and Unions throughout the World; and, as such, must adhere to the myriad of "Privacy" Laws throughout the World.
Hence, "Privacy" is one of the reasons that we cannot "Share" the "Living" individuals/persons in our own "Private Spaces" - it is NOT the ONLY reason; but, certainly has a bearing on the matter.
Another factor is that such would have to be 'Coded'/'Programmed' into the Programme that is "Family Tree" of 'FamilySearch'.
'Yes', these "Living" individuals/person in "Family Tree" of 'FamilySearch' WILL, in fact, in many (most) cases will be "Duplicated" - unfortunately a necessary situation that currently cannot be avoided.
I know that there are "Duplicates" of the "Living" ME - at least, one for my Wife and each of our Children, not to mention my other ("Living") extended Family members.
Here are some "Knowledge Articles" in 'FamilySearch' in regard to "Private Spaces" and "Living" individuals/persons:
Who can see my living relatives in Family Tree?
How does Family Tree protect the privacy of living people?
What Family Tree features are available in my private space?
Can my living relatives share a Family Tree private space to work together?
Can I transfer my Family Tree private space to another user?
Here is a good place to start, with how things work in "Family Tree" of 'FamilySearch' (and, other programmes as well):
Getting Started with Family History
Here are the main things you need to know as you get started
How to Use the Family Tree
What is The Family History Guide?
The Family History Guide
About The Family History Guide
Note the record you add for you living spouse - will not be the same record that your living spouse has on her account. (if she has an account) - even thouigh it represents the same person.
which I wonder if this weas your real underlying question.
Let su know if you have reoslved yoru question - based on the helpful info from Brett,