Below you can see the screen with mission mode selected (actually on two settings). You could do the same for land.
That is how this is done in main and v1.13. In older versions you could map a channel to a particular mode - but the QGC configuration was hard for users to understand and added little value, so that was removed.
It depends on the version you are using of PX4. On main you can follow the instructions here to map an RC channel to an actuator channel, and you could assign that actuator channel to main 8 pin.
On release builds you can do the same thing but it is harder. YOu would follow the instructions here using the headings about “mixer allocation” (rather than control allocations): Payloads and Cameras | PX4 User Guide (v1.13)