Data Validation from a Dropdown list with additional settings

Question

I have a workbook that I'm trying to amend to validate what is being entered based on the text entered + another field that has already been entered.

So the users would enter ABC in B5, then as they type into A8 I want the validation to be ABC + Whatever is typed.

I have the following VBA today:

    Option Explicit

Private Sub Worksheet_SelectionChange(ByVal target As Range)
'Update by Extendoffice: 2018/9/21
' Update by Chris Brackett 2018-11-30

Dim xWs As Worksheet
Set xWs = Application.ActiveSheet

On Error Resume Next

Dim xCombox As OLEObject
Set xCombox = xWs.OLEObjects("PartsCombo")

' Added this to auto select all text when activating the combox box.
xCombox.SetFocus

With xCombox
    .ListFillRange = vbNullString
    .LinkedCell = vbNullString
    .Visible = False
End With


Dim xStr As String
Dim xArr


If target.Validation.Type = xlValidateList Then
    ' The target cell contains Data Validation.

    target.Validation.InCellDropdown = False


    ' Cancel the "SelectionChange" event.
    Dim Cancel As Boolean
    Cancel = True

    xStr = target.Validation.Formula1
    xStr = Right(xStr, Len(xStr) - 1)

    If xStr = vbNullString Then Exit Sub

    With xCombox
        .Visible = True
        .Left = target.Left
        .Top = target.Top
        .Width = target.Width + 5
        .Height = target.Height + 5
        .ListFillRange = xStr

        If .ListFillRange = vbNullString Then
            xArr = Split(xStr, ",")
            Me.PartsCombo.List = xArr
        End If

        .LinkedCell = target.Address

    End With

    xCombox.Activate
    Me.PartsCombo.DropDown

End If
End Sub

Private Sub PartsCombo_KeyDown( _
                ByVal KeyCode As MSForms.ReturnInteger, _
                ByVal Shift As Integer)
    Select Case KeyCode
        Case 9  ' Tab key
            Application.ActiveCell.Offset(0, 1).Activate
        Case 13 ' Pause key
            Application.ActiveCell.Offset(1, 0).Activate
    End Select
End Sub

Here is an image of the sheet: