Fetch records

Hi,
I'm new to this and it's my first post in this forum.
I'm struggling to get data from a related form into a field. It's something to do with the lookup value being in a lookup dropdown I think.

Form A (Room_Cat_Form) has fields called Room_Usage and CtFm_Net.

Form B has a lookup field for Room_Usage and I need the corresponding value of CtFm_Net in another field (Usually_Net).
I tried this but it just returns the Record ID.

input.Usually_Net = Room_Cat_Form[CtFm_Net == input.Room_Usage];

It's the easiest thing in the world to do on other platforms. I only need this answer once to unlock lots of other things.

thanks