Let us suppose that you want to call a process automatically.
You can create the following diagrams:

And you can execute the following code:
&ProcessDefinition = &Server.GetProcessDefinitionByName('BPDStarter')
&ProcessInstance = &ProcessDefinition.CreateInstance()
&ProcessInstance.Start()
&ProcessInstance.ThrowSignal('S1')
commit
return
Notes
The Signal Intermediate Event needs to be active to listen to the signal.