Some older versions of RangerFlex (pre-v3.0) did not provide a means to set endorsement text via the user interface. However, you can set the endorsement text manually by editing Ranger's CapiOptions.ini file.
Add the following section to the CapiOptions.ini file and enable endorsing via the RangerFlex user interface:
[RangerFlex]
RearEndorseLine1=Line 1 text
RearEndorseLine2=Line 2 text
RearEndorseLine3=Line 3 text
RearEndorseLine4=Line 4 text