I know I need to create a link for the language override and it should include &return=, but I have no idea how to structure the URL.

You won't be able to include &return= as that needs to be a base64 encoded URL of where you want to return them to, which you won't be able to do dynamically from a Custom HTML field.

Can you please advise me where to look to show me how to create a link that takes you to login and then the inbox?

There's no such link available. Have added a feature ticket to change the redirect on the messages page to just send to login instead of home page.

forge.joomlapolis.com/issues/7379

There's 2 notification email bodies. 1 with the message included and 1 without. The [message] is not unused for that string.

So, it is not possible to create a link to take people directly to the inbox or PMS via login, if needed?

It's possible to take them directly to their messages, but if they're not logged in it'll give them an access permission error and send them to the home page. Use the below URL to link to their messages page.

index.php?option=com_comprofiler&view=pluginclass&plugin=pms.mypmspro&action=messages

The feature ticket above I'll be changing that redirect to send them to login instead of home and set the &return= for the login URL so it sends them back to their messages after logging in. Should make that experience more use friendly.