How can I add a source to someone in Family Search that I found on a different web site?
If that is the source of the information, please add it. Be sure to fill in all the fields you can such as Where the Record Is Found or Describe the Record so those who cannot access the record because it is at a subscription site can know what it is.
For example, North Carolina 1847 birth record from the North Carolina Births 1840 to 1860 found at Ancestry.com and the URL. There are more professional ways to write sources, but if you don't want to figure that out, just give all the information you can so that another researcher knows what you looked at and where. They can possibly find North Carolina Birth 1840 to 1860 at another repository.