Populate TextBox based off ComboBox selection

I need to populate a textbox based off 2 combobox selections. My first combobox selects the worksheet. "08BOG, 09BOG, 10BOG...15BOG" My second combobox lists majors based off of the worksheet selected in combobox 1. I need my textbox to populate the value 18 rows over the selected worksheet and the selected row.

Currently I have something like this

Private Sub CommandButton1_Click()
With Me
.TextBox1.Value = Sheet1.Cells(.ComboBox2.ListIndex + 19, 18)
End With

End Sub

But obviously this is all wrong because it is only referencing one sheet instead of the selected sheet in the combobox. SOMEBODY PLEASE HELP