Which preposition to use with forma

for Occurrences 2%

Manual of court practice, with rules and forma for all Indiana courts.

at Occurrences 1%

As I could not absent myself from the office longer than ten days at a time without permission of the government at Washington, I had to return pro forma at that term, when, to my surprise, I found my apartment in possession of a stranger.

Which preposition to use with  forma