pass output of js function into PHP form

donate.php

<form method="post" action="confirmation.php" name="donationForm" id="donationForm" onsubmit="return validateAmount();">

<input type="hidden" id=
    "my-frequency" name="frequency" value=
    <?php echo '<script type="text/javascript"> myLastFrequency(); </script>'; ?>
    />
    
    <button class="new-look-select-color new-look-submit-btn" data-pointer-submit="true" name="sbmit" type="submit" value="Submit">SUBMIT
	  		</button>

myjs.js

function myLastFrequency() {
        lastFrequency = document.getElementsByClassName('active btn btn-frequency-custom');
        if (lastFrequency.length == 1) {
          return lastFrequency[0].getAttribute('data-freq');
        };  
      }