TYPO3 Core

TYPO3:
  CMS:
    Form:
      persistenceManager:
        allowedFileMounts:
          100: 1:/sadf/
          110: 1:/asdf
          120: 1:/asdf/
          130: 1:/asdfas/
          140: 1:/asdfasf
          150: 1:/ergvqerw/
          160: 1:/refqerfn/
          170: 1:/qfewfqwe/
        allowSaveToExtensionPaths: false
        allowDeleteFromExtensionPaths: false
        #allowedExtensionPaths:
          #10: EXT:example/Resources/Private/Forms/
      formManager:
        dynamicRequireJsModules:
          app: 'TYPO3/CMS/Form/Backend/FormManager'
          viewModel: 'TYPO3/CMS/Form/Backend/FormManager/ViewModel'
        stylesheets:
          100: 'EXT:form/Resources/Public/Css/form.css'
        translationFile: 'EXT:form/Resources/Private/Language/Database.xlf'
        javaScriptTranslationFile: 'EXT:form/Resources/Private/Language/locallang_formManager_javascript.xlf'
        selectablePrototypesConfiguration:
          100:
            identifier: 'standard'
            label: 'formManager.selectablePrototypesConfiguration.standard.label'
            newFormTemplates:
              500:
                templatePath: 'EXT:dgwerg/Resources/Private/Yaml/NewForms/EventForm.yaml'
                label: 'Veranstaltungsformular'
      prototypes:
        standard:
          formElementsDefinition:
            Form:
              renderingOptions:
                templateRootPaths:
                  100: 'assets/templates/ext/formframework/Templates/'

Please use something like

100: 'EXT:assets/templates/ext/formframework/Templates/'

My template is not in an extension, that‘s why I have no path with ext:.
But nevertheless, I also tried EXT:extname/Resources/Private/formframework/Templates/, no luck.

(In Templates/ I have Finishers/Confirmation/Confirmation.html)

Here is the solution (finishers is different):

      prototypes:
        standard:
          finishersDefinition:
            Confirmation:
              implementationClassName: 'TYPO3\CMS\Form\Domain\Finishers\ConfirmationFinisher'
              options:
                #message: ''
                #variables:
                templateName: 'Confirmation'
                templateRootPaths:
                  100: 'path/Resources/Private/formframework/Templates/Finishers/Confirmation/'

I think form ext needs a documentation update.

The documentation already exists https://docs.typo3.org/typo3cms/extensions/form/Config/proto/finishersDefinition/finishers/Confirmation.html#options-templaterootpaths.
Your initial question refers to the form element templates which can be configured with

TYPO3.CMS.Form.prototypes.<prototypeIdentifier>.formElementsDefinition.Form.renderingOptions.templateRootPaths

Ok :)
Thanks!