Option Explicit

Sub a_b_c_v0()
Dim JachList1, JachList2
Dim indx As Long, i As Long, r As Long: r = 1
Dim shL1 As Worksheet: Set shL1 = ThisWorkbook.Sheets("List1")
Dim shL2 As Worksheet: Set shL2 = ThisWorkbook.Sheets("List2")

    If Trim(shL2.Cells(1, 1).Value) <> "" Then
        JachList2 = Split(shL2.Cells(1, 1).Value, ",", -1, 1)
        indx = UBound(JachList2)
        
        Do Until Trim(shL1.Cells(r, 1).Value) = ""
            JachList1 = Trim(CStr(shL1.Cells(r, 1).Value))
            
            For i = 0 To indx
                If JachList1 = Trim(CStr(JachList2(i))) Then Exit For
            Next
            
            If i > indx Then shL2.Cells(1, 1).Value = Trim(shL2.Cells(1, 1).Value) & "," & JachList1
            
            r = r + 1
        Loop
        
    End If
    
    Set shL1 = Nothing
    Set shL2 = Nothing
End Sub