FolderItemDialog.ShowModalWithin
From Xojo Documentation
Method
FolderItemDialog.ShowModalWithin(Parent as Window) As Folderitem
Supported for all project types and targets.
Supported for all project types and targets.
Displays the FolderItemDialog as a Sheet window on macOS within the window specified by Parent.
Notes
If the application is not running under macOS, ShowModalWithin is equivalent to ShowModal. Returns a Folderitem in Result if the user validates the dialog; otherwise Result is Nil.