You need a text box on your userform, not a list box, then you can use the following code, which assumes default names for the form, the text box and the button,
Option Explicit
Private Sub CommandButton1_Click()
Selection.Cells(1) = TextBox1.value
Hide
End Sub
Private Sub UserForm_Initialize()
With TextBox1
.EnterKeyBehavior = True
.MultiLine = True
.Text = Selection.Cells(1).Text
End With
End Sub
and call the form with the following macro
Option Explicit
Sub ShowForm()
With UserForm1
.Show
Unload UserForm1
End With
lbl_Exit:
Exit Sub
End Sub
Wouldn't Alt+Enter be altogether simpler?