<form id='main' method='POST' action="https://ya.ru">
  <input type='text' name='test'  required>
  <a href='#' onclick='onClick()'>Click</a>
</form>
<script>
  function onClick() {
    var mainForm = document.getElementById('main');
    
    if (mainForm.checkValidity()) {    
    	mainForm.submit();    
    }
  }
</script>