Question How to Choose what output the audio interface should use??

Aug 3, 2019
I have a Scarlett 2i2 ( I know it is bad) and I have a a pair of Yamaha hs8 connected to them.

After working with bands I understood that an easier solution for hearing everything while recording was a headphone amplifier.
I bought one and connected it to the interface but whenever I play a thing on my Daw (Fl Studio) only the monitors play.

How do I make the interface change the output to the headphone amp?
When I disconnected my speakers the amp worked just fine. but I am sure there are easier ways to change the output the do not involve disconnecting cables.
So, How do I make my audio interface channel the audio to the headphone amp rather then the monitors?
The line/monitor output of the Scarlett has been split and you connected both the speakers and hp amp to it?
Both or either should work depending on which are turned on.
If there is a problem driving one when the other is off, which can happen, you will need a input selector which you would use in reverse.