Hello, @Roger Roger,
Welcome to the Microsoft Q&A platform!
If you need to add permissions for a large number of users at the same time, I have tested the command you provided and it does what you need. If you encounter any other issues, you can provide the error code and screenshots, which will help to resolve your issue more efficiently.
If you don't have many users that you need to assign permissions to, I suggest you do it directly in EAC, it will be easier and faster. Here are the steps to provide users with Send As/Send On behalf permissions to the DL in EAC.
1.Log in to EAC and click Recipients>Groups>Distribution List.
2.Click on the target DL and select Settings>Edit manage delegates.
3.Add delegates for the target users according to your requirements.
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Thank you for your support and understanding.
Best Wishes,
Alex Zhang