What is the specific Islamic will that every Muslim must bequeath before their death?

A will (wasiyya) is a disposition of an estate after death, and it includes:

Obligatory Will (al-wasiyya al-wājiba): That to which an obligatory right of others is attached, such as the payment of debts. Recommended Will (al-wasiyya al-mustaḥabba): Setting aside a portion of wealth for charitable causes with the intention of reward. Forbidden Will (al-wasiyya al-muḥarrama): A will for something forbidden, or with the intention of causing harm.

A person's will regarding a portion of their wealth is not obligatory according to the majority of scholars; rather, it is recommended for closeness to God and seeking reward. As for a will concerning obligatory rights, it is obligatory. There is no specific format for a will; rather, any expression that indicates the testator's intention suffices.