OK, I realize if you know nothing of wiring this is confusing. Use any source from your fusebox that’s a switched circuit you want. You can do this because whatever you are powering will only draw as much as it needs (unless there is a short). The 10 amps rated part is just what your “add a circuit” piece can handle due to wire thickness. Your gauge probably won’t even draw an amps worth of current. So if you really want to be anal, put like a 5 amp fuse in the “add a circuit” slot.