How do I wire my inline duct fan to my Goodman model gmp100-4 gas furnace fan so that the inline duct fan comes on and off when the furnace fan does?
I understand that I need a relay from the inline duct fan but don't know where to connect the relay to the furnace.

I assume you only want to run the fan in air conditioning, correct?

If you chose to have the duct fan run at all times when the fan in the furnace ran then one method would be to install a sail switch (as air blows through the duct it pushes against the sail and closes the switch) in the duct work , the switch would then make the circuit to the fan. You could also use a pressure switch that would bring on the fan when there is static pressure in duct.
I mention these ways because most furnaces have two or more speeds and just wiring to the fan relay would not bring it on in heat mode,