How to pre-fill fields from values in a URL query string

Created by Phil Kurth, Modified on Fri, 01 Jul 2022 at 09:29 AM by Phil Kurth

If you need to use values from the URL query string — e.g; example.com/page?my-custom-param=some-value — you may do so using the af/field/prefill_value filter.

add_filter( 'af/field/prefill_value', function ( $value, $field, $form, $args ) {
  // If we need to run this only on a specific form, check for the form key.
  if ( $form['key'] !== 'YOUR_FORM_KEY_HERE' ) {
    return $value;
  }

  // Check the for the desired field and if the URL contains the expected parameter, use it.
  if ( $field['name'] === 'YOUR_FIELD_NAME_HERE' && isset( $_GET['YOUR_QUERY_VAR_HERE'] ) ) {
    $value = $_GET['YOUR_QUERY_VAR_HERE'];
  }
  
  return $value;
}, 10, 4 );

Whilst the above example works for most field types, some field types will require special formatting:

Pre-filling group fields
Pre-filling clone fields