We are remodelling our bathroom and need to mortar bed a whirlpool tub.
The bath is on the second floor and there is a hole in the floor where the drain goes beneath subfloor. How do we set the mortar and keep it from going down the hole?

Are the second floor trusses or joists up to the task to carry the full weight load that tub will result in? I know its late in the game to present that but better late than never.

You can create a dam of sorts. Like with a big sponge with a hole cut in the center you glue down is just one possibility.

Did you sister the floor joists or consult someone before installation? There is a lot of floor load with tub, water and people. As far as the hole goes. Get a small piece of PVC, bigger than the hole, cut it to the motrar depth and set it around the hole. Put a finish nail to keep it from moving. Pour your mortar, let is sit a couple of hours, pull the nails and the ring and you should be good to go.

I assume you are setting a jetted bathtub not a true whirlpool or Jacuzzi tub and want to build a dam around the drain opening to prevent the mortar or thin set from running down to the ceiling below.

I'd get two or three toilet wax rings, open then up into one long piece and press them to the subfloor around the opening.

OK sounds like you are setting the tub in a bed of mortar, right? Make your mortar stiff and it will not run, you can also use other materials like Structolite to set a tub. You had a tub before, I assume, so its likely your bath can handle the weight.