I did this 2 weeks ago, I (gold) am booked on a cash CE fare and my partner (bronze) is on an avios redemption (obviously under a separate booking ref).

Simply called the gold line, explained I'd like to link the booking and requested 1C for my parter on the outbound and the return, no issues at all.

I do not know the 'correct' procedure but it seems to have worked for me.

CB