Some mandatory parameters are missing ("user") to generate a URL for route in drupal 8

Using drupal 8, in my account page I had a menu with view like custom_orders. In that page when I click to add new order, new form "user/{user}/custom_orders/add" will be opened. After submit that form page should be redirect to "user/{user}/custom_orders" this page.

I tried some,

custom_form:  
  path: '/user/{user}/custom-orders'
  requirements:
    _user_is_logged_in: 'TRUE'
    _access: 'TRUE'
    user: \d+
  options:
    no_cache: TRUE

And in form page, I have redirected like

$form_state->setRedirect('custom_form');

But it was not working.

I have already tried this, Some mandatory parameters are missing ("user") to generate a URL for route but it was not working for me.