Snap Creator simply passes commands onto the systems command interpreter, so you will need to figure out a command string that you can call from Windows to execute the PowerShell command.
I suspect that you will need to call powershell.exe then add && to tell Snap Creator to run the next command within PowerShell (so the command is passed as a single line)
Here is a similar example that is calling a Teamcenter shell instead of a PowerShell...shell:
C:\PROGRA~1\Siemens\TEAMCE~1\tc_menu\TcEng_config1.bat && backup_modes.exe -u=infodba -p=infodba -g=dba -m=normal
Once the command works with your variables, you can add in the Snap Creator variables and include the command into Snap Creator's PRE and POST commands.
You need to be sure to add any commands that you are calling to the allowed_commands.config file on the Agent. You also must restart the Agent to get the allowed_commands to be accepted.
In the example command above I have to add the following to allowed_commands.config:
Snap Creator knows these are two different commands, so they must both be added.
Since backup_modes.exe isn't fully qualified when we call it, it doesn't have to be fully qualified in the allowed_commands.config file either.
There is a video on adding allowed commands here: https://www.youtube.com/watch?v=niZYjI710_Q
I also have a video coming on using the APP_QUIESCE and APP_UNQUIESCE commands in Snap Creator. That video is recorded, but need to be edited and reviewed.
The concept should be similar to what you are trying to do.
Hope this helps,