We investigated on this and indeed this seems to be an issue of our "Custom Actions" page UI. A fix for this issue will be available in a future version of our product, thank you for bringing it to our attention. When the fix will be released we will update this forum thread.
Until then, as a workaround, you can use our "Table Editor" feature (available only in our Enterprise and Architect edition of Advanced Installer).
Just go to "Table Editor" page and select the "CustomActions" table. Select the row of your OpenUrl action and set its "Target" column like this:
Code: Select all
Then save and rebuild your setup project.
Hope this helped.
All the best,