Use single quotes for the sheet name just as you would for standard formulas.

[VBA]Sub snb()
ComboBox1.List = Filter([Transpose(IF('CTC-Form'!BJ12:BJ21="","~",'CTC-Form'!BJ12:BJ21))], "~", False)
End Sub

Private Sub UserForm_Initialize()
snb
End Sub
[/VBA]