Pase el parámetro a la función JavaScript OnClick
<input type="text" id="dt" value="" required oninvalid="setCustomValidity('Please enter e value to process!!')" oninput="setCustomValidity('')" >
<input type='button' value="Save" onclick="processRequest(document.getElementById('dt').value)">
<script>
window.onload=(()=>{document.querySelector("input[id='dt']").value=(new Date()).getFullYear();});
processRequest=((val)=>{
alert(val);
});
</script>
Simon Chirambira