How to assign shared mailbox to azure account?

Hi All,

In our 9.2.1 environment, we have configured both the connector for Azure AD and Exchange Online.

We are having problems directly assigning a shared mailbox to Azure AD accounts.
We are trying to assign them via system role with an Azure group, i.e., we have created the groups as outlined here on Microsoft's site:

https://learn.microsoft.com/en-us/exchange/recipients-in-exchange-online/manage-mail-enabled-security-groups

and then we do assign groups on the system role but we find that the group is only readonly. Can anyone tell us how to solve this problem and if we are following the right path?
The goal is to assign people automatically and massively without having to go by hand and assign each person send as etc.

Thanks a lot