Right now, the target page is defined as the fourth parameter in the parser function call. This does not allow custom page destinations to be defined. I propose that this information be moved to the form configuration page.
Subpage Mode
This would create a text box above the button allowing a custom subpage name to be defined.
{ "steps": { ... }, "target": { "mode": "subpage", "rootname": "DestinationPage/" } } `
Same-Page (Append) Mode
This would not allow a custom subpage to be made.
{ "steps": { ... }, "target": { "mode": "append", "pagename": "DestinationPage" } } `
With this, we could drop the fourth ParserFunction parameter. I also propose dropping the fifth one since I'm not sure it actually does anything.