Short Answer: NOTHING, just leave the "Event" as "Blank".
Of course, UNLESS, there was a "Common Law" component; or, they "Lived Together".
There are many such "Couple" Relationships in "Family Tree" of "FamilySearch" with NO "Event", just as there is in life.
An "Event" is NOT a mandatory requirement in "Family Tree" of "FamilySearch".
I hope this helps.
you could add a note indicating the person was not married.
but most people just leave the marriage record empty - and thats it.
in the relationship
you can choose various options such as "common law marriage etc,"
you an also add a note to the relationship - explaining the circumstances
if they were not married - lave date blank.