custom lookup insertion via api

custom lookup insertion via api

hi,

i am trying to insert custom filed( look up) via name and it gives error 4401Unable to populate data, please check if mandatory value is entered correctly.
here is my code
<FL  val="Co-emprunteur 1">test user</FL>

is there a way to insert it using id? am i missing some thing here?
FYI: the name i am passing to this tag already exists in contacts.