one of my whatchamacallits

I have a form where the insurance company wants the name of a beneficiary and this person's relationship to me. I filled in the name and then stopped. "Sweetie" seems too informal for insurance and responsible financial business like that. "Partner?" "Domestic partner?" Can that be appropriate for someone I don't live with, someone who is married to another person? It occurred to me that this may well be a solved problem among polyfolk. Do people use girlfriend or boyfriend in this sort of context? Or just "friend?" I like the way "friend" can cover so much, but I know some people use it to minimize or understate intimacy. And of course if I use "chosen family," the clerk transcribing my form into the computer will have to grumble and check "other," no matter how many categories they had programmed.