If you obtained what you want from the interface, you just have to follow in the code how it was done.
First you need to create a "full source file" (matrix ImageGridAmp with zero everywhere except where your simulated signal is), and then multiply it with the forward model.
Put a breakpoint at the beginning of the Run() function in process_simulate_recordings.m, start the simulation from the Process1 tab, and when the debugger stops start executing line by line the code, exploring the contents of the variables at each step:
https://github.com/brainstorm-tools/brainstorm3/blob/master/toolbox/process/functions/process_simulate_recordings.m#L82
If you are not familiar with the Matlab debugger, search first for a tutorial about it, this is a very powerful tool to follow code execution.