Any proposed ordinance may be submitted to the governing body of the municipality by a petition signed by qualified electors thereof equal in number to fifteen percent of the votes cast for all candidates for the executive officer at the preceding regular municipal election. The petition must be filed in the city auditor’s office and must contain a request that the ordinance set out in the petition be submitted to a vote of the qualified electors of the city if it is not passed by the governing body of the municipality.