Error in C#, passing text from program.cs to form 1

You have to use an instance of Form1 to access its textBox1 control.
The way you wrote it, is like you are expecting the textbox to be a static element of the Form1 class, which it obviously is not.
One way to do that is to keep a reference to the instance at the time you create it (most probably, in the Main method of the Program class):

static void Main(string[] args) {
  // ...
  Form1 form = new Form1();
  form.textBox1.Text = "Whatever...";
  Application.Run(form);
}

PS: I just saw you have reposted your question instead of editing already existing one. Please do not repost the same question several times; it will not help you getting better answers.

static void Main(string[] args)
{
    // Create an instance of Form1.
    Form1 form = new Form1();

    // Modify the textbox value on it.
    form.SetTextBox1Value("Whatever...");

    // Show the application form.
    Application.Run(form);
}

    public void SetTextBox1Value(string text)
    {
        TextBox1.Text = text;
    }