The relay is there to handle a higher current than your swith will/will not handle that's all.
Use a multimeter and find the wire on the ignition that gose +12 when the key is fully turned round into the starting position. Then need to link the feed to ignition to this wire with your switch (or relay) in the middle of it.
standard:
ign feed--------[ign barrell switch]---------to starter motor solenoid
what you want:
ign feed--------[your switch]--------------to starter motor solenoid
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