Here's how I do it.

Open the Simicons Panel (SHIFT+2), then select the SSR icon.

And using the attached image as a guide:

(1) Move the M-3/A switch to 3/A
(2) Move the Master Control to NORM
(3) *Turn the Mode D ON
(4) Make sure that the CIV/MIL switch is set to CIV
(5) Enter the squawk code

The button/knob/switch names are from the ‘C-130 X-PERIENCE’ FLIGHT MANUAL Part II –Systems and Equipment (csc130_manual2.pdf), page 24, under the heading PANEL ‘X’ - IFF/SSR TRANSPONDER CONTROL PANEL.

* This might need to be "Turn the Mode C ON" (I can't remember how I saw this one was supposed to be set (it was a long time ago), so it could be Mode C, or Mode D). :-/

Note. I don't know if this works with any of the online virtual airlines as I never did a flight in any of them.

---