おめでとうは、congratulations (複数形）という表現が最も一般的。２つ目の例文のように Please accept my congratulation on ... のような使い方では、単数形でも使われることもある。 昇進は、promotion to ... というフレーズが良いでしょうが、be promoted to ... でも良い。また、例文のように、it is with pleasure や it is a pleasure などでスタートする文章で、お祝いの言葉を述べるスタイルも良く見かける。 さらに、please accept my (our) best wishes for your success というフレーズも覚えておくと便利。

Congratulations on your recent promotion to General Manager. I know how hard you have worked to earn the recognition you presently enjoy at ABC Corporation.

Please accept our best wishes for your success in your new position.

Yours truly,

Dear ＿＿＿＿

【昇進のお祝い】

It was with great pleasure that I learned your promotion to the position of General Manager with ABC Corporation. Your firm has made a very wise choice, and I am sure that you will excel in your new role as General Manager.

Please accept my congratulation on your promotion and my very best wishes for your continuing success.

Warmest regards,