§ 13-a. Moneys for maintaining the conference of mayors and other city officials of the state of New York and any of its activities. The common council of any city is hereby authorized to appropriate and expend annually, from moneys raised by taxation in such city, a sum to meet the actual and necessary expenses of maintaining and continuing the conference of mayors and other city officials of the state of New York and any of its activities, in this state, for the purpose of devising practicable ways and means for obtaining greater economy and efficiency in the government thereof. The moneys thus appropriated shall be raised by tax on the real and personal property liable to taxation in any such city in the same manner as other city expenses.