You don't, you just use a fixed percentage of whatever business tax they pay to fund open source development.

How are you going to separate out companies that profit from open source software, from those that don't? Would the fixed percentage be same for a company that is built 100% on open source software as it would for one where a handful of people in one department occasionally use VS Code to write some python scripts?

If you want to fund open source using tax money, then fund open source using tax money. Trying to implement a special Open Source Tax and apply it only to those that "profit from" open source seems both counterproductive and virtually impossible.

I think everybody profits from open source, since a large part of the Internet runs on open source, so there is no big need to discriminate.

So why make a special separate tax that has to be calculated and administered? Just use the tax money you already collect to fund open source projects.

If you want to spend more money you have to increase taxes. I don't mind whether we just increase an existing tax or create a new one.