We need to replace the standard command (the New button) to open our customized session.

We can open our session using command
choice.add.set:
before.choice:
start.session (...)

But this does not override the standard command and as a result we have two sessions open: standard and our new custom session.

We need to open our new session only, without the standard one.

Any suggestions?

Use choice.again() in the before.choice section, in your case:

choice.add.set:
before.choice:
start.session (...)
choice.again()

This way you run the custom session and stop the stand. command from excecuting after it.

Great! Thanks!