As far as I can see from their site, StreamYard does not support OBS.
StreamYard is not on the list of streaming platforms OBS is configured to send to.
You could configure it as a Custom destination if StreamYard provided the information, but it appears as if they are promoting this as a service that works within a browser and are not supporting dedicated streaming clients like OBS.
The only thing I can think of would be to open a full screen projector for OBS' program window on a display, and capture that display in StreamYards using whatever methods they support.