Get value of programmatically (form_alter) added fields at form submit
A field added in the form_alter hook is not created on the entity. Therefore you cannot retrieve the value in either hook_entity_presave or hook_entity_insert.

What you want however you can do by creating an additional submit handler for the form.

function _MODULE_NAME_custom_submit_handler($form, FormStateInterface $form_state) {
  $email = $form_state-&gt;getValue('email_field_name');
  // send email.
}


You can also get the saved entity (which does not have the email field) using $form_state-&gt;getFormObject();