@Musmuharam, Thanks for posting in Q&A. Based on my researching, I didn't find the built in policy to enforce startup apps. But I find we can change the registry key to set the startup app.
You can test the registry key on one affected device. If it can work, then we can try to write PowerShell script to set the registry key. After the script is working. Then we can consider deploy the script via Intune to bulk change it.
https://learn.microsoft.com/en-us/mem/intune/apps/intune-management-extension
Hope the above information can help.
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