You found the manual - or some kind of - but seen not to have read it carefully, as it says:
MULTIPLEX receivers supporting SRXL
All MULTIPLEX receivers which are telemetry capable can be configured to send out a SRXL signal. An exception are the MULTIPLEX FLEXX receivers, which are sending the SRXL signal by default without any receiver configuration required.
So there is nothing you can do as the SRXL signal is offered by default.
It may well be that despite the default setting the system is not running to your satisfaction. The reaseon can be that yout application does not meet the requirements. Meanwhile there are two versions of the SRLX-signal, V1 and V2. But as you can see above: you can't chance the version as the RX-Flexx supports only V1.