Hey. Currently variant sudokus cannot be created from within the app, but can be imported. You can create them with any of the existing tools out there, such as https://sudokulab.net/ and https://www.f-puzzles.com/.
I can't seem to figure out how to do this? I tried clicking "Import Puzzle", pasting the link to my puzzle (https://f-puzzles.com/?id=2cxkv7s4) and it says "This link does not contain valid share data". Same thing when I try pasting the non-shortened URL.
SudokuPad only supports sudokupad links, not f-puzzles. So convert it first (by replacing "https://f-puzzles.com/?load=" with "https://sudokupad.app/fpuzzles"). Then you should be able to press "Play in SudokuPad" at the top of the page.