How to pass the form element to Ext AJAX request

I have a problem with passing a form to an ExtJS6 AJAX request. The docs state:

form : Ext.dom.Element / HTMLElement / String

The form Element or the id of the form to pull parameters from.

Scope is not the problem, this references to Ext.form.Panel

My code is:

Ext.Ajax.request({
  scope: this,
  method: 'POST',
  url: 'someurl',
  form: this.getEl(),
  isUpload: true,
  success: this.someSuccessFunction
  failure: this.someFailureFunction
});

I have also tried this.getId(), this.getForm().getId() and this.

Whatever I try, I get this error:

Uncaught TypeError: form.submit is not a function

Does anyone know what is the problem with my code?

Ext.form.Panel does not contain form-tag. This feature is used in case of real html forms. Have a look the request of the following example: fiddle

Ext.application({
name: 'Fiddle',

launch: function () {
    Ext.create('Ext.button.Button', {
        text: "Submit",
        renderTo: Ext.getBody(),
        handler: function () {
            Ext.Ajax.request({
                url: 'user.json',
                form: Ext.fly('userForm').dom,
                success: function (response, opts) {
                    var obj = Ext.decode(response.responseText); // THIS IS A REAL HTML FORM
                    console.dir(obj);
                },

                failure: function (response, opts) {
                    console.log('server-side failure with status code ' + response.status);
                }
            });

        }
    })
}
});