You could cheat, and use a label.
From the control toolbox, draw a label and set its BorderStyle to 1 -
fmBorderStyleSingle. Set its font to Wingdings and set its font size to
whatever best fits inside the label. Give the label the following code
in VBA:

Private Sub Label1_Click()
If Label1.Caption = "ü" Then
Label1.Caption = ""
Else
Label1.Caption = "ü"
End If
End Sub

(if you have named the label something different then subsitute Label1
with whatever you named it).

If you use this method, then, when referencing the label in code, if
Label1.Caption = "ü" then it's ticked, otherwise it's not ticked.
However, if you use this method, you cannot use the keyboard to operate
it.