@Belinda - Can you please confirm which version of AEM is used as I remember return type of function was changed. Can you also try to remove it and add again?

@Belinda - Can you please confirm which version of AEM is used as I remember return type of function was changed.

@Belinda- Output Type of service can't be changed from FDM (Form Data Model). You need to implement following workflow Create custom action/service in dynamics which store contact and return Id.Include custom action in FDM and use it instead of default POST contact/contact then remaining follow should work