Even this over three year old blog post is still valid: you don't have to create the user variable before assigning it, use the X button next to the dropdown list and you'll be able to create that user variable. Good idea because I don't like the labeling of the generic variable because it has the same name as the TEB, and almost all the time duplicate labels do cause conflicts. This is one of the exceptions.
Save Time: Create Associated Variable for Text Entry Box while Defining this TEB - Captivate blog