Forking, composer and github project

I want to fork yii2 to do custom developments, so I followed the composer instructions at

But yii2 is separated in 2 repositories, yii2 and yii2-framework, and composer pulls from yii2-framework, but we are suposed to do pull requests in yii2.

Forking yii2-framework and using the configuration from composer works (forking yii2 doesn’t), but now how will I do a pull request to yiisoft/yii2?