I have two custom dialog boxes (plus the required ones ExitDlg, FatalErrorDlg, etc.), the first one sets a property using an Edit control and the s
ExitDlg
FatalErrorDlg
Rather than scheduling the custom action in the InstallUISequence you can publish it on the button click:
InstallUISequence
1 1
EDIT: The Publish element's condition needs to explicitly evaluate to true to run, so add "1" as the text of the Publish elements.
Publish
"1"