Common Market -- hold back "x" percent of goods ea month

I see applications to nearly all common markets require a state enterprise to hold back each month "X" % of the goods it produces.

Where can I find this setting to change it to say "hold back 25%" of goods produced, so I can at least apply to one or more common markets?

If you want to offer the same percentage for all your corps, it's easy. Go to the menu at the top of your screen, where it says Common Market. Then click on "Contracts," then click on "Offer Contracts for Products your Corporations produce."

Scroll to the bottom of that page, where it says "Offer % of the total production," and fill in the blank.

But if you want a different percentage for each corp, there is no function for that. On that last page, you would need to individualize the amount of product offered for each corp.