Make a formula url field.
Use something similar to this (your variables will change)
URLRoot() & "db/" & [_DBID_TABLE_2] & "?a=API_GenAddRecordForm&_fid_11=" & URLEncode ([Field I want to Pre fill])& "&_fid_8=" & URLEncode ([another field I wand to pre fill])& "&z=" & Rurl()
If you don't want the redirect back to the origin, remove the & "&z=" & Rurl()
at the end.
If you have more info of what you are trying to have it right, and some fields that need to be written to, let me know and I can help with syntax if needed.