Adding name of the sheets of one workbook to combo box of another workbook

I have 2 workbooks test1 (has 3 sheets) and tests(has 1 sheet).

In tests I have a combo box named ComboBox1.I have to add the name of the sheets from workbook test1 in this ComboBox1.

I have written the following code

Sub Workbook_open()

Dim sht As Worksheet

Set wkbSource = Workbooks.Open("test1.xls")

For Each sht In wkbSource.Worksheets

ActiveWorkbook.Worksheets(1).ComboBox1.AddItem sht.Name

Next sht

End Sub

It is giving me an error

ERROR : Object doesn't support this property or method on line ActiveWorkbook.Worksheets(1).ComboBox1.AddItem sht.Name

How can I fix it ?