give-save-custom-field.php

<?php
/**
 * @param $value
 * @param $option_value
 */
function my_special_field( $value, $option_value ) {
    ?>
    <tr>
      <th scope="row" class="titledesc" colspan="2">
        <input type="text" name="my_special_field" value="<?php esc_attr( give_get_option( 'my_special_field' ); ?>" />
      </th>
    </tr>
    <?php
}
add_action( 'give_admin_field_my_special_field', 'my_special_field', 10, 5 );

/**
 * Save the field
 */
function save_my_special_field() {
    if ( ! isset( $_POST['my_special_field'] ) ) {
        return;
    }
    
    give_update_option( 'my_special_field', sanitize_text_field( $_POST['my_special_field'] ) );
}
add_action( 'give-settings_save_gateways', 'save_my_special_field' );