Where do you set outdial rules?

I need to have the system add an extra 8 since my Asterisk Server connects to another pbx to get out. Both require an 8 to dial out and right now the only way is to dial two “8”'s

I would like to set it up to just dial one 8 if possible…

add 8+ (or is it just “8”?) to the Outbound Dial Prefix in the trunk setting under Trunks.