See Issue #88667
FormEngine had the feature to add additional submit handlers via the option
be set by form element renderables. TYPO3 uses RequireJS and a rewritten FormEngine since version 7, the property
Additional, functions of
TBE_EDITOR that are associated with that feature (namely
addActionChecks) were removed as well.
The option has no effect anymore, the code won’t get executed at all.
All 3rd-party extensions using this option are affected.
It is possible to create and register an AMD module.
$resultArray['requireJsModules'] = 'TYPO3/CMS/MyExtension/SubmitHandler';