Accessing a forms textbox value

Question

Hello friends.
I know there are lots of topics about this subject, but i couldn't find an answer for me.

I have a Program.cs file includes main. I'm defining a variable, and i want to show that in Form1. And user will be able to change that variable's value.

So, how can i write and read TextBox1 on Form1, from main program?

Thank you for your helps.