Local Market Trade (Little Upsilon)

How do you contract on local market without the ceo,but using your own state corporations?Such as bread company,I am currently buying 100 quality supplies and i would like to trade contracts through the local market for wheat to increase the quality?Could some one please explain this how it works.Thank you.

If you want to buy all the bread yourself, there's a button on the company page for "Contract full production to and accept".