https://api.silverstripe.org/4/SilverStripe/Forms/TreeDropdownField.html#method_setHasEmptyDefault That's probably what you want then. You need to make sure the select has an empty option
Do you mean that you want to be able to have nothing selected?
Then you just need to add the right code to your page template. Where that goes specifically will depend how the site is built, but it will be in the templates directory, possibly in
http://Page.ss or maybe in a footer include or similar
You need to decide how the popup is to be triggered (bear in mind, automatic pop-ups are hated by most web users)
OK. https://getbootstrap.com/docs/4.3/components/modal/ Bootstrap can provide the pop-up, etc. out of the box
Is the site built using a front-end framework (eg. Bootstrap, Foundation, etc.) ?
OK, that's even simpler then
OK, so you want a form.
OK, and what will be in the popup?