Did you manage to solve your problem? I have the same! I'm creating a form that allows register user to submit their articles but I want to be able to select the category that they will save their article. Is it possible? Feel free to ask any information...
Hello. You can use the field Article Category ID (jForm - Category) to assign an article to a category. If you want to choose the default category for the user's article, select the #2 on the right hand side, and using the double arrow in the live value column, select the category you want as the default. If you don't want the user to see the drop down, then select Hidden as the variation for that field on the site form.
You should also add the Category ID field + Category ID Value to your LIST type if you want articles showing only from that current category. If you have more types using more categories you could find articles from other categories appearing in your list if you have not added this.
I have discovered that my query was to do with permissions. You have to be logged in for the field to do it's thing. Public users are not invited to the party. My original post is below if you are a beginner like me and are for some reason interested in my ramblings