input type=text but the webpage in the background makes an api call (and due to the nature of the middleware that api call is quite slow) and when the api call completes, the element gets replaced from input to select which contains only valid values (i'm praying that the user will fill the form slowly enough for the api call to complete before they get to that form element, or if the api call fails the user wouldn't really notice that and fill the form as usual, except they would have to write out the thing manually and not select it)
disabled for input and make it say "loading..." (i think it would be prettier than a simple text and also i think that the arrow of the dropdown list appearing only after the api call finishes would be a neat way to communicate to user that they can interact with the list)