The code in the designer class is the same class as the code in your form class.
It's just split across two different files.
Therefore all the members in one file are directly accessible in the other file.
'
If you have a label called label1 on your designer (which is reflected in Form.Design.cs) you just access it in Form.cs code as
label1.Text = "foo";
like that.
You shouldn't need to mess with the designer code directly. You're probably engaging in some kind of anti-pattern.
What are you trying to do?