install schluter flange to trap

Hi all

I want to install a Schulter flange for my shower and the instructions says I need to press it down on the mud bed.
However my ABS drain pipe is flexible and goes down upon applying pressure. How do I ensure the flange is well connected to the pipe?

Note my subfloor is still not in place, thus I still have access to the drain pipe.

Thanks

You can use a cat claw to reach down and "grab" the ABS pipe and prevent it from pushing down.

If you can figure out what height the trap should be at to allow you to bed the drain properly, you could add some blocking to prevent it from moving, or strap it.

After I replaced the trap, I was able to put some blocks under it.

Just curious, if I were to use a cat claw, do I grab by putting it inside the pipe?
Outside will be hard given schluter has such a big flange.