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

$validators = $form_state->getValidateHandlers(); 
$validators[] = 'mymodule_form_validate'; 
$form_state->setValidateHandlers($validators); 

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