C# to insert data in textbox and to display that data in combobox

As far as I know, Access is irrelevant unless you are going to store the info in a database and retrieve it in a different form - which is not the most obvious or efficient way to do it!

You want to create a property on the second form to accept the name:

public string UserName { get; set; }

Then set the value before you display the Form2 from Form1:

Form2 f2 = new Form2();
f2.UserName = myTextBox.Text;
f2.ShowDialog();

In the setter of the UserName Property, you can set the ComboBox value - and use the getter to retrieve the currently selected item.