Hello, joefarage, if your aim is to apply the retention tag to the shared mailbox, you should add the tag to the retention policy and after it, you will have to apply it to the shared mailbox. The steps are the following:
1) Log in to Exchange Admin Center using the administrator account.
2) Go to compliance management, retention policies and add a new retention policy. Fill in the Name field and add your personal retention tag to it.
3) Go to recipients > Shared > Select your shared mailbox and edit it. Add your retention policy to this shared mailbox like here.
Also, I recommend visiting this helpful link, if you want your important Microsoft 365 data to be recoverable in any unpredictable situation.