Form Submit without module Is a module required to submit a form ?
I get a standard form from our designer and try to send it as follow:

&lt;form action="/kontakt" class="sky-form" method="POST" name="kontakt"&gt;
  ...
  &lt;input type="submit" class="btn btn-u btn-u-lg" value="Nachricht absenden"&gt;
&lt;/form&gt;


I have tried different function calls without success

MYTHEME_form_submit(array &amp;$form, FormStateInterface $form_state)
contact_form(array &amp;$form, FormStateInterface $form_state)
contact_form_submit(array &amp;$form, FormStateInterface $form_state)
MYTHEME_contact_form_submit(array &amp;$form, FormStateInterface $form_state)
...


i have to say, that i am a beginner in Drupal 8, so help is very appreciated.

thank you.
Two ways to approach this.

One way is to use the Webform module. If this is just a simple contact form, you can get this done with Webform in five minutes.

The second way is create a custom module that implements a route, Form class and related methods. Wouldn't take an intermediate long, but a beginner should try Webform first.

You cannot simply paste form HTML into a twig file and expect it do anything. Drupal is ignorant to its existence and therefore no hooks will work, as you already saw.

https://www.drupal.org/project/webform