Here cboFromSheeet is the name of the combo box.
Dim wbFrom As Workbook
Set wbFrom = Application.Workbooks.Open(lblFromFilePath.Caption)
Dim ws As Worksheet
'Clear the combo box
cboFromSheeet.Clear
With cboFromSheeet
For Each ws In Worksheets
.AddItem ws.Name
Next ws
End With
cboFromSheeet.Text = "Select a Sheet"