How I can add Meta Keywords in Subdomains?

I want to add meta keywords to my subdomains which are different from my root domain.

If you are using WP, you can definitely add those through an SEO plugin like Yoast - but you will need to manually enable that option, as meta keywords are now considered a deprecated feature on the search engine - least likely to be taken into consideration after the keyword spamming incident may years back.
There may be instances where this feature may not be available on the latest Yoast (they may have removed that), but you can try another like SEO Ultimate.