I leave it blank (hopefully to be added later) . Do the same for a husband that marries one of my line that I don't have one
of his names, sometimes it's the given sometimes it's his surname.
I put the husband's last name using double brackets. This gives me at least a clue of the family relationship when I'm looking at a name list.
Likewise, if I don't know a first name. For instance, if a daughter, Jane Doe Smith, is widowed and has moved back in with her parents in a census listing, you will know her married name, but not her husband's first name. So I will name him [[Husband of Jane Doe]] Smith.
The advantage of this is that you can exclude this information from reports very easily.