Hi Kaishi,
Following this previous Community post where DJ was able to get the search bar to direct to a vertical, I believe you just need to have the verticalUrl URL parameter for redirectUrl and you don’t need the verticalKey parameter.
Additionally, I just want to be sure for [[domainname]] you are using the search results page URL and not the main domain URL. For example, this https://www.yext.com/answers?verticalUrl=products.html, not https://www.yext.com?verticalUrl=products.html without the answers.
Can you see if this works for you?