Here is a quick tutorial explaining how to assign specific categories to submitted content. For example, you have three categories and three forms. You want the submitted posts for each form to go to its assigned categories. Here is how to make it happen.
Let’s say that you have a form for which all submitted posts should be assigned to the category “donuts”, which has a category ID of “99”. Easy. Just add the following hidden field anywhere in the form:
<input name="usp-cats-default" value="99" type="hidden">
Change the value “99” to match the category ID that you would like to use and done. Multiple categories work too, just separate multiple IDs with commas, like so: