Hi @BianceBee ,
The name of the tagged user cannot be obtained directly through a function expression.
Because only the notified_user_id is provided in the output of the message.

You can get the username by adding actions. I made a sample for you.
Post a message in Yammer and notify a user.

Create a flow, and I will explain it step by step.

Get first message.

Use the ID of the notified user to get the user information and create a new item with the user name.
outputs('Get_all_messages_(V2)')?['body/value']
items('Apply_to_each')?['notified_user_ids']
items('Apply_to_each')?['notified_user_ids']

Save and run flow.

Best Regards,
Wearsky