How to write an effective introductory letter to introduce a HR consultancy firm?

I would suggest checking out the answers and subsequent links on the following links.

https://www.askmehelpdesk.com/technical-writing/sample-letter-introducing-upcoming-magazine-96215.html

https://www.askmehelpdesk.com/human-resources/introduction-letter-human-resources-recruitment-company-78287.html

How to write a email to a hr consultancy