Asaf,
You can do one of two things:
First, and easiest, is to go to the form designer of Form1, and change the
text box's Modifier attribute to internal or public.  Then you would use the
following code in Form2 to reach Form1's text box:
((Form1)this.ParentForm).txtTextBox1.Text = "abc";

Secondly, you can expose the text property only of Form1's textbox via an
internal or public property.  In Form1 add the following:

public string TextBox1Text  // or something more explanatory
{
  get { return txtTextBox1.Text; }
  set { txtTextBox1.Text = value; }
}

Then in Form2 you would access the text box value using
((Form1)this.ParentForm).TextBox1Text = "abc";

The second method just provides more protection to your form1 textbox.  Only
the text of the textbox can be changed and no other properties can be
changed accidentally.  This is the method I recommend.

Trevor B