Hi Sean, sorry for my confusion, now I understand, yes, you have OSPi (I was judging from the picture you posted that you have an OSDIY).
So, I have to clarify to you that only recent OSPi Plus boards (version number 1.42) have added the pin headers for RF transmitter. What you have is probably the original version which comes with a yellow relay and not RF transmitter headers. Since a couple months ago, we’ve decided to retire the relay, but instead add RF pin headers.
The firmware by default uses the same pin (originally assigned to relay) for the RF transmitter. So I think the easiest way is probably to pick a different GPIO pin (GPIO18 is recommended), and modify the firmware to match your new pin. Check the attached image for suggested pins. Basically you can connect RF transmitter’s DAT, VCC, GND pins to GPIO18, +5V, GND pins marked in the image. (although the transmitter can work with 3.3V, use 5V helps boost its transmission range). Next, modify the PIN_RF_DATA defined in defines.h (under the #if defined(OSPI) section) to 18 (originally it’s 15, which is the same as the relay pin). Then recompile the firmware. This should allow you to use RF transmitter. Let me know if this is clear.