Tax Code §6.22

(a) The assessor and collector for a taxing unit other than a county or a home-rule city are determined by the law creating or authorizing creation of the unit. (b) The assessor and collector for a home-rule city are determined by the city's charter and ordinances. (c) The governing body of a taxing unit authorized to have its own assessor and collector by official action in the manner required by law for official action by the body may require the county to assess and collect the taxes the unit imposes in the county in the manner in which the county assesses and collects its taxes. The governing body of the unit may revoke the requirement at any time by the same official action.