Call a PHP function from submit

Hi,
I have a function in functions.php

function updateHasReadFlag($user) {
  // I added support for using this function either with user ID or user object
  if ( $user && is_int( $user ) ) {
    $user_id = $user;
  } else if ( ! empty( $user->ID ) ) {
    $user_id = $user->ID;
  } else {
    return;
  }
  return update_user_meta( $user_id, 'GDPR_TERMS_READ', true );
}

Now I want to call this function on click of the Accept button from the GDPR pop up.

<form method="post">
            <?php if ($isshortcode == 1) : ?>
            <input type="hidden" name="wptp_popup_id" value="<?php echo $termspageid; ?>" />
            <?php endif; ?>
			<div class="tthebutton">
				<input class="termsagree" name="wptp_agree" type="submit" value="<?php echo $tagree; ?>" />
			    <input class="termsdecline" type="button" onclick="window.location.replace('<?php echo $termsRedirectUrl ?>')" value="<?php echo $tdisagree; ?>" />
			</div>
		</form>

How can I do that?