maybe a select case statement would work well in this case?
Private Sub CommandButton1_Click()

Select Case ComboBox1.Text
    Case "MHT"
        Worksheets("MHT").Range("C15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT").Range("H15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT").Range("C35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT").Range("H35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT").Range("C55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT").Range("H55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
    Case "MHT2"
        Worksheets("MHT2").Range("C15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT2").Range("H15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT2").Range("C35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT2").Range("H35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT2").Range("C55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT2").Range("H55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
    Case "MHT3"
        Worksheets("MHT3").Range("C15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT3").Range("H15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT3").Range("C35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT3").Range("H35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT3").Range("C55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT3").Range("H55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
    Case "MHT4"
        Worksheets("MHT4").Range("C15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT4").Range("H15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT4").Range("C35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT4").Range("H35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT4").Range("C55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT4").Range("H55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
    Case "MHT5"
        Worksheets("MHT5").Range("C15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT5").Range("H15").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT5").Range("C35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT5").Range("H35").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT5").Range("C55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
        Worksheets("MHT5").Range("H55").Value = "PROD " & TBProd.Value & " - " & "MHT " & TBmht.Value
End Select