As said before, the tunnel URL is not permanent and changes. The desktop app would use it automatically as external services would otherwise be unable to send webhooks to your local n8n instance. I don’t think there is any way currently of changing the desktop app behaviour with regards to using this tunnel, but I’m sure @ivov can confirm.
That said, n8n desktop listens (by default) on port 5679 of your local machine, so you can replace https://$tunnelName.hooks.n8n.cloud/ with http://localhost:5679/ to address your local instance directly instead of using a tunnel.