change fields in lookup wizard on the fly

Good morning, everyone

I would like to do the following:
I am making a reservation system with following tables

Table room: number, size, floor
Table reservation: number, name, date

Now I'm making an add page for a reservation. For the field room I do a Lockup Wizard. There I would like to show only those rooms which are still free on the added date on the add page.

I think that's only possible on the fly.

Can someone help me please