get_query_var

/**
 * Register custom query var.
 *
 * @param string[] $public_query_vars The array of allowed query variable names.
 * @return string[]
 */

function acme_login_query_var( $public_query_vars ) {
  $vars[] = "sample_query";
  return $vars;
}

add_filter( 'query_vars', 'acme_login_query_var' );


/**
 * Check if query var is exists on login.
 *
 * @return void
 */

function acme_check_query_var() {
  // null
  var_dump( get_query_var( 'sample_query' ) );

  // ez működik
  var_dump( $_GET['sample_quer'] );
}

add_action( 'login_init', 'acme_check_query_var' );