169.625.  Board of trustees may adopt plan requiring all contributions by employee to be paid directly to retirement system. — The board of trustees by regulation may adopt a plan wherein each and every employer included within the retirement system shall specify that contributions withheld from compensation to members are being paid directly to the retirement system by the employer and that the members had no option to receive the contributions directly.  These contributions shall be credited to member accounts as required by law, shall be included in the individual member salaries reported to the retirement system, and shall in all ways be considered member contributions.