Sample 1:
Congratulations on your recent promotion to Regional Manager.
I know how hard you have worked to earn the recognition you presently enjoy at (name of firm) , and I feel that they are very wise in having made their choice.
Please accept our best wishes for your success in your new position.
Sample 2:
It was with great pleasure that I read of your promotion to the position of Regional Manager with Lucky Ltd.
I am sure your firm has made a very wise choice and that you will excel in your new role as Regional Manager.
Please accept my congratulation on your promotion and my very best wishes for your continuing success.
source: 4hb.com