A letter asking for a collaboration letter from a compnay or can be a proposal for the same

I just started working as a HR for a company, my first task is to write a letter/proposal to our foreign clients, to get a kind of collaboration from them in whcih it should be listed that they have been working with us from last xx years and they are happy working with us and also satisfied with the quality of work.

PLEASE SOMEBODY HELP ME, WHAT SHOULD I WRITE........

THANKS IN ADVANCE

GURPREET SINGH