I want to create a subscription plan with a category named [username] and a group "My Group" in the category [username].

From the plugin I can make a second auto_2 but obligatory (combo box) there have to select a category and I want to set [username].

Can't be done through the integration in this case. There's no way for the integration to predict a future creation of a category. The only reliable way to link a group to a category is with the category ID and at this point in time your category does not yet exist. In this case you'd need queries, but please keep in mind no triggers will fire for your groups creation.