How do I specify whether to send a CAN bus message on low-speed or high-speed CAN?
The following description may vary depending on each manufacturer, but concepts are similar. 
Basically a vehicle works with two physically separated CAN networks, which is the high CAN, sometimes refered as C-CAN, and low CAN, sometimes refered as B-CAN. The high CAN usually runs at 500kbps and connects powertrain ECUs, while the low CAN runs at 125kbps and connects media/cluster and other systems that are not safety relevant. The OBD is often not connected to any of above, it can be a third bus connected to the system's gateway: the body. The last is a centralized ECU that is connected to all CAN buses and can act as a CAN router or have its own processing. Normally if you send messages through the OBD it won't reach its destination because the body can act as a firewall to prevent unwanted data getting into its systems. But there are cases in which the OBD connects to a particular high/low bus, not very safe though. You firstly need to find out which bus you are connected to and if this bus has access to whatever ECU you are trying to reach. A good approach is trying to listen to your bus. If no messages arrive you are probably connected to the body which is blocking all your messages.

WARNING: Sending messages through the vehicle's buses can cause unpredictable behavior and safety risks