Portal: Where to find Template for Custom Rush

I want to create a Hardcore Rush server that starts with 4 players, not 8 minimum. My understanding is, I go to Portal -> Create new -> Standard.

But there I cannot select a pre-defined game mode. Where do I find a JSON file that has the normal Rush code in it please?

As written on the website below, you have to create a custom server and create the whole logic from scratch - there are no JSON files anywhere?

Source: https://www.bombrats.com/rush-breakthrough-mit-echtem-teamplay-spiel-mit-battlefield-6/