Yes, this is possible in a TextBox, but the problem you have is that you'll need to show the UserForm modelessly because you need to select cells in a worksheet while the Userform is on show. If you want to do exactly as you describe, ie the very next keystroke is '=', then you will need to re-activate the UserForm otherwise you'll simply add an '=' into your selected cell.
I believe the Show command won't re-activate a modeless UserForm that is already on show and I'm not aware of an Activate command for a UserForm. Others may well know of one, but I'd use a couple of APIs to do the job. So the code in your Userform could be as follows (you may need to adjust the API declarations if you have Win64 and/or VB7 https://msdn.microsoft.com/en-us/library/office/ee691831(v=office.14).aspx):
Option Explicit
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" _
(ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function SetFocusAPI Lib "user32" Alias "SetForegroundWindow" _
(ByVal hwnd As Long) As Long
Private mHwnd As Long
Private mTextBoxHasFocus As Boolean
Public Property Get TextBoxHasFocus()
TextBoxHasFocus = mTextBoxHasFocus
End Property
Public Sub AppendText(appendedText As String)
'Add the text to textbox
TextBox1.Text = TextBox1.Text & appendedText
'Activate this userform window
SetFocusAPI mHwnd
'API doesn't trigger UserForm_Activate event so call it
Me.ActivateByCode
End Sub
Private Sub TextBox1_Enter()
mTextBoxHasFocus = True
End Sub
Private Sub TextBox1_Exit(ByVal Cancel As MSForms.ReturnBoolean)
mTextBoxHasFocus = False
End Sub
Public Sub ActivateByCode()
'Set focus on any another control then back to textbox
btnSave.SetFocus
'Set cursor at end of text
TextBox1.SelStart = Len(TextBox1.Text)
TextBox1.SelLength = 0
TextBox1.SetFocus
End Sub
Private Sub UserForm_Initialize()
'Acquire the handle for this window
mHwnd = FindWindow("ThunderDFrame", Me.Caption)
End Sub
You'd then call the AppendText routine from a WorkSheet_Change event or, as in this case, a Workbook_SheetSelectionChange event (which caters for selections of a cell in any WorkSheet):
Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
If UserForm1.TextBoxHasFocus Then
UserForm1.AppendText Target.Address
'Uncomment the line below if you want sheet name as well as address
'UserForm1.AppendText Sh.Name & "!" & Target.Address
End If
End Sub
And remember to show your UserForm modelessly:
UserForm1.Show False
If you want to cursor to be in the TextBox when the UserForm opens, then add this line too:
UserForm1.ActivateByCode