I'm not too familiar with that setup so this may not be much help, but a quick look at the Fx3u-20ssch manual seems to indicate they use buffer memory addresses to control the servo.
I'd suggest you download the manual. The manual I downloaded was JY997D21301M. Starting on page 228 it shows the monitor data buffer memory areas and on page 240 starts the Control data which I think you'll be most interested in. I can't give you a straightforward answer on what M bit they used because it will depend on programming. There should be some instruction to transfer the buffer memory to M bits in your program. For your question of M2065 I'd look to see the closest M bit (that is = to or below M2065) that has an instruction similar to --[Mov k4M20xx U0\Gxxxx] if it is a status bit or Mov U0\Gxxxx K4M20xx if it is a command.
It looks like you use FX configurator-FP to set up the servo. I don't have this software, but if you do there may be something there that will help. I know for Q series using their setup tool (simple motion tool) under the help tab there is an excel file that gives a list of all buffer memory addresses.