A bequest(G) to the Society may be best thought of as a deferred donation.
In its normal form, a bequest comprises a paragraph inserted into a new will, or formally attached to an existing will, that states your desire to assist the work of the society by a donation to be made following your death.
You may bequeath a fixed amount or any part or percentage of your assets to the Society. Your financial or legal advisor can help to draft the paragraph so that your wishes are made clear and your instructions carried out.