You can achieve this by creating a batch file that first runs your PowerShell script and then starts the Macrium backup.
1. Create a batch file (.bat) with the commands to run your PowerShell script and then the backup.
2. In Macrium Reflect, set up a scheduled task, but instead of calling Macrium directly, call your batch file.
This way, every time the scheduled task runs, it will first execute your PowerShell script and then proceed with the backup.