Show entry in dynamic dropdown without using the 'selected value' field

I have a dynamic dropdown in a modal to let the user select some let’s say categories, using an ‘add’ button next to it in case the desired category is not yet listed which then should automatically be selected. Adding a new category and getting the identity value back from the query works fine, but:

I’d like to set the dropdown to the new entered value and I’m struggling there. When using the ‘selected value’, it works but all new entries from this point on use the identity value by default instead of blank. When I try to set the value of the select using dynamic behaviours without using the dynamic behaviour, nothing happens.