Using a form alter hook, we can add additional submit handlers to a form. The proper way to do this is to load the current submit handlers, add the new one to the array, and reset the validators in the form state:

$submit_handlers = $form_state->getSubmitHandlers(); 
$submit_handlers[] = 'mymodule_form_submit'; 
$form_state->setSubmitHandlers($submit_handlers ); 

First, we will receive all of the currently set submit handlers from the form state as the $submit_handlers variable. We then append a new callback to the end of the array.
Once the $submit_handlers variable has been modified, we will override the form state's submit handler array by executing the setSubmitHandlers method.