How can one write a letter from a recruitment agency to client informing the client that you have a suitable candidate for the opening that they have in their compnay

Are you a new employee of a recruitment agency?
It seems so simple to write this letter. You also save copies of formats for all letters (these days usually emails) and just change details as needed.
A business letter must be very short.
If you don't have a name of a person, you write To Whom it May Concern:
You write 2 sentences about who you are and how you charge (or reference a separate page of rates).
2 more sentences about having an excellent candidate and what that person's specific skills are.
How you can be reached.
(Possibly one sentence thanking them for consideration of your letter.)
Paragraph breaks between each of the above topics.
Sincerely yours,
You

Don't expect a response. I would guess that there might be about a 3% chance. Most companies who are willing to pay a recruiter have done so already, or don't want one.
If your job pays on commission, find another job.