If I understand your requirement correctly, your use case is similar to:
Click on "something", a "screen" is popped up. Inside it, there are multiple check boxes for user to select.
If this is correct, the implementation can be the following:
"something" for user to do initial click can be either a link or button. The popped up "screen" can be an overlay which contains multiple check boxes in a repeating grid.
In the link (or button) configuration, you select 'Overlay' as the Target, the local action supplied to the link configuration will invoke a section which contains a repeating grid, each row has one checkbox or multiple check boxes.
Please check OOTB Overlay, Grid and Checkbox examples in UI Gallery (Designer Studio -> User Interface -> UI Gallery) for detailed information.
Could you please provide more clarity regarding the requirement. Not sure which control are you referring to by "multiple check boxes selection dropdown". Is it check boxes, multi select or a dropdown ?