The Marketing Cloud Organization ID is supposed to populate automatically on the add a tool screen. This only happens when you are logged in with an ID that is linked back to your Marketing Cloud user.
If your not seeing this happen, It could be something with the way your DTM user is setup.
With that said, you can always manually enter the organization ID. To find the ID you would need to
Log in to marketing.adobe.com
Click on the solution switcher in the upper right
Click on Administration
View the organization ID at the bottom of the page
If you can't see the administration link, you likely aren't listed as an Admin for your organization. If this is the case, or you'd like to have someone look into your user's linking with DTM further, I'd suggest contacting Client Care.