How to Get Notifications from Gmail ‘Updates’ Tab

Like the comments mention, you can't drag and drop into the Primary tab unless you want to always categorize emails from that forum as "Primary"

What you could try doing is setting up a filter that automatically stars an email from the forums email address (e.g. forum-group@example.com). This will also allow you to refine which forum emails are starred (e.g. only contain the words "lorem ipsum" etc)

Once you get it so that Gmail stars email from your forum, click on the Gear menu and select Configure Inbox

From there you can select the option to add any Starred email to your primary inbox. This should allow you to keep the forum categorization and still have it show up in the Primary tab.

It is not possible to add your own tab, you can only choose: Social, Promotions, Updates, Forums etc. designed by Google.