How can I activate a floorplan and get the users choice to the server?
Create a form on a page with a submit button and add to the form the floorplan shortcode with the type parameter set to either checkbox or radio.
E.g. you can add this with the Custom HTML block in the editor:
[floorplan id=84 type=checkbox]
<input type="submit" value="Submit">
Warning: this example should not be used in production! For instance, it is highly recommended to use a nonce. Therefor it is worth considering using a form builder plugin.