Have on your form a button named Command1 and a textbox named Text1.
Code:
Private Sub Command1_Click()
Text1.Text = "" 'Set the textbox to empty
Text1.SetFocus 'Give the textbox focus so we can type in it
SendKeys "Hi" 'Type into the textbox
End Sub
There are a few other "better" ways of accomplishing this but they require complicated things such as APIs and Hwnds therefore I felt it best not to complicate things for you at the beginning. Hopefully you understand this and keep going further.