Steps:

Sign in to the Microsoft 365 Admin Center.

Go to Exchange Admin Center (or EAC).

In the left-hand menu, click on Recipients, then Shared.

Click on the + (plus sign) to create a new shared mailbox.

Fill in the required fields, such as the name and email address of the shared mailbox.

Once created, assign users who need access to the shared mailbox by clicking on Edit next to the shared mailbox.

Under Members, click + (plus sign) and select the active users you want to grant access to.

Click Save.