https://bugs.winehq.org/show_bug.cgi?id=35522
--- Comment #10 from François Gouget fgouget@codeweavers.com --- (In reply to Dmitry Timoshkov from comment #9) [...]
But in the simple case it could indeed provide a default value if that field is left blank.
Once again, this isn't different from a multi-page UI.
The difference is that in the multi-page UI the TestBot knows what's in the dif before showing you the Test Executable field, whereas in the single page case it only has access to the diff not only after putting up that field, but also after you click on the Submit button.
I think you can work out the UI implications from there.