So in MSFS from the main menu, go to the Options, then Control.
You should see your different input hardware on the list, your Thrustmaster HOTAS would be visible as well. Select that hardware and it will show all the bound commands. By default, this is only showing the assigned commands, which is the ones that’s already bound. It’s not showing all of the possible commands in the sim.
On the left side of the screen, you should see the filter, and it’s showing “Assigned”. You need to change this to “All”. And the filter will show every single command that’s available in the sim. So you’ll see a lot of commands and they’re empty as they’re not bound to any button yet.
On the left side on the filter, you should also see a search function as well. In the search box, just type in “Hold Throttle Reverse Thrust” and you should see the command shown on the list, just empty without any bound buttons.
Click on that “Hold Throttle Reverse Thrust” command, and it will bring another box. Click on the empty box in the middle to start the binding, then press the button on your Thrustmaster HOTAS that you want to use to hold for reverse thrust. You should see the “Joystick Button #” in the box. Just click OK, and apply and save the control. And that’s it, that button is now bound to the hold throttle reverse thrust command.