How to create a jQuery autocomplete select with the possibility to add/enter a new element when no matching entry exists?

How to create a jQuery autocomplete select with the possibility to add/enter a new element when no matching entry exists?

Hello, I´m quite new to jQuery but still hope to be in the right topic ;)

My plan is to use a select element in a FORM thats populated by a JSON file. I want that select element to "assist" the the user in an AJAX way by showing only options, that start with the so far entered characters.
If there´s no option that suits the input, the entered data should be sent with the rest of the form data to a sccript that further processes the data on the server.
I know, there´s a way to populate the options with the JSON data and that I can use the combobox for the search but where are the values from the selctions stored and if the entry is a new one, where will this entry be stored to be read out to be processe?
Thank you for your help.