The convention is to name the upstream remote ~origin~. If you follow
this convention, then you have to do nothing else and the remote by
that name is automatically used, provided it exists and regardless of
whether other remotes exist. If it does not exist, then no other
remotes are tried.
If you do not follow the naming convention, then you have to inform
Forge about that by setting the Git variable ~forge.remote~ to the name
that you instead use for upstream remotes. If this variable is set,
then Forge uses the remote by that name, if it exists, the same way
it may have used ~origin~ if the the variable were undefined. I.e. it
does not fall through to try ~origin~ if no remote by your chosen name
exists.
This is from the manual, but I will have to move (or copy) to a more prominent place within the manual. (This is currently in the node titled "Token Creation".)