I’ll assume you are using the steps outlined in the HowTo for adding BACnet network objects to your project, including setting up a Client Mapping to your external control setpoint.
I’m not very familiar with the details of the Scheduler object you need, but looking at BACnet Configurator I can see that the “small” version of the object only has one writeable property - Out of Service. I suggest changing to either the “medium” or “large” version of the Scheduler object. This opens up lots of other properties with read and write access.
