Can you provide the serial number so I can see what is available?
Pretty sure you will need to install the proportional roller wheel on the right hand joystick to get variable flow if you want to lock it at anything less than maximum flow. The on/off button only turn the auxiliary solenoids on fully to provide maximum flow.
I'm looking deeper into it but you will definitely need a momentary contact switch to provide a ground to the correct pin at the auxiliary ecm. If the ground is there for more than 30 seconds you get a fault.
You can get the(NNN) NNN-NNNNcontinuous flow/interlock override switch from a CAT dealer. It should install in the rearmost switch position on the upper left panel. There will be orange, pink, white, yellow, black, and purple wires to it in the panel. The P993 orange goes to the ecm to enable continuous flow. It is just grounded when the switch it pressed. You activate the function then press the switch which sends a ground signal to the ECM.
You should be able to simulate that without the switch by grounding that wire in the panel while the auxiliary button is pressed. Any switch can do that. Just remember you will get faults if it is grounded more than 30 sec.
If you want to go to the proportional control handle to modulate auxiliary flow, you will need to have a dealer or someone with CAT ET software involved because the software need to be reconfigured to allow the proportional control. If you want to go that route, I can provide the part numbers.