Done a bit of searching but did not come across anything on modal.
This is quite surprising as I just entered
Modal in the search bar and many results popped up.
Aside from that, your terminology is not correct. It is not a ‘popup Form’ but rather a ‘Dialog’.
Non-Modal is determined by the Parent being set. Modal has the form (or document) as the parent and Non-Modal has the DeskTop as the Parent.
Now you can create a dialog in two ways - use the IDE to create one graphically or create one entirely through code. Each of these can be used in a non-modal fashion but each requires macro coding.
The following are links (possibly containing other links) with further information and there are samples:
Calc BASIC: How to popup a dialog ?
how to compose dialog invisibly?
how can i set DesktopAsParent for a dialog in a library?