import { Controller } from 'stimulus'
import Rails from "@rails/ujs"

export default class extends Controller {

  static targets = ['user', 'form']

  submitForm(){
    Rails.fire(this.formTarget, 'submit')
  }

  onPostSuccess(event) {
    console.log("success!")
  }
}

===================================

import { Controller } from 'stimulus'

export default class extends Controller {

  static targets = ['form']

  submitForm(){
   this.formTarget.submit()
  }

  onPostSuccess(event) {
    console.log("success!")
  }
}