If you're like me you have only written macros till now. I was so used to opening Menu>
Tools
Macros
Organize Macros
LibreOffice Basic...
that I didn't even notice the menu choice right below it for Organize Dialogs...
where you can create and edit dialogs.

When you open it up you get among other things the list of the largely undocumented LibreOffice Dialogs
which is all read-only for some reason although it appears you can edit a dialog's properties. This is also where you can open My Dialogs and Dialogs for your other opened documents.

Be sure Menu>
View
Toolbars
Toolbox
is checked which gives you a menu of drag and drop tools.
Select something on the Dialog. Small green corners will appear. Also properties will appear with General and Events tabs.

The Events tab is where you link the Dialog to macros to create behavior. It seems you can't drag to move, but you can set the xy coordinates to adjust the Dialog. Also there is a preview button
to preview your dialog. (Use Escape to close it again.)
When Regina writes in another answer here, "To see such dialog in action, simple run "StartChangeUserfields" in module Userfields in library "Gimmicks", the way you do this is to go to Menu>
Tools
Macros
Run Macro
Then drill down in LibreOffice Macros Gimmicks click Userfields and then StartChangeUserfields and hit the Run
button.

Which gives you this Dialog which you can use to edit your use data.

Verify that this works by entering something like in Company name, and clicking Select
below. Then to go a main LO window, and see Menu>
Tools
Options
LibreOffice
User Data
.