OK, that will still be tricky with a native glide form. It sounds like what you really need is a relation and lookup columns to retrieve the Registration and Model from the table that contains that information based on the Unit number. Like I mentioned in my earlier post, you could choose not to show the Registration and Model in the form, but only get the Unit from the user. In the table that you write the form results to, you could put your relation and lookup columns there. Then you can get the information from another table based on that unit number without actually having to write the information to the table. The reason you can’t show the matching vehicle in the form is because the relation won’t work until the form is submitted.
Instead I would consider using a custom form. It’s more work to build, but it would allow you to enter the Unit, then immediately show the Registration and Model from the resulting lookup columns, and pass them through an add row action on a button in the custom form. It’s hard to explain the whole process, but I’m confident that it would work once you learn how a custom form works.