First Introduce yourself
Second say what she has done to deserve a promotion Say something like I think she is a really valuable member of the team or she is really skilled at her work
Third end the letter by saying something like thank you for your understanding your „under your name here“

This should provide a rough outline for your letter feel free to change something if you want
spam fails_task lang_mismatch pii not_appropriate hate_speech sexual_content quality toxicity humor helpfulness creativity violence