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".)