How does Clan Castle troop donation work in clan wars?

The clan castle in your war base is different from the clan castle in your village and must be filled separately. The troops donated to a war base will be used to defend it against all attacks during battle day.

Unlike your regular village, there is no need to request troops for your war base. Instead, all war bases are automatically open for troop donation during preparation day. To donate, simply tap on a clan mate’s war base during preparation day and donate troops directly. Remember, troops cannot be donated to war bases during battle day.