How to call/submit Form from another page in custom module?
You can do it with the FormBuilder::submitForm() function.

To register a new user (taken from the docs):

// register a new user
$form_state = new FormState();
$values['name'] = 'robo-user';
$values['mail'] = 'robouser@example.com';
$values['pass']['pass1'] = 'password';
$values['pass']['pass2'] = 'password';
$values['op'] = t('Create new account');
$form_state-&gt;setValues($values);
\Drupal::formBuilder()-&gt;submitForm('user_register_form', $form_state);


You might have to include the namespace for the FormState class before this code if not done already.