About this letter
An offer letter tells a candidate what you are offering — the role, the pay, the joining date, the place of work and any probation — and how long they have to accept. It is the start of the employment record, so keep it short and accurate; the detailed terms belong in the appointment letter and your policies.
This is the same starter wording CoreStack People uses for offers, where the candidate signs online with a one-time code sent to their email and the signed copy is filed against them.