You may want to get some information back from your SEO as to why they are recommending that. Do you have a certain search engine that's popular in your part of the world that still uses it?
If Google is the target, meta keywords are not useful.
Agreed with Jason. Metas are not useful now!
So no need to add meta keyword tag but if still, you want to add, contact your website developer and ask where will you get this option if developed else ask him to develop.
We focus on Google to get business and we also don't have meta keywords on our website (https://online-paystub.com/) still we are performing well in google :)