arcmap editor attribute window fill value programmatically

In my map I have a layer named PARCEL, when create new feature on this layer, the attribute window will open automatically. There are 2 attributes in this layer: PARCEL_ID and SUB_ID.

Since the PARCEL_ID is the left part of SUB_ID, I intend to display the PARCEL_ID value in SUB_ID field after user input the PARCEL_ID value, this is to minimize user key in wrong information.

Is there a way to do this without a customized editor tool?

I am using vb.net arcobjects 10.2.2.

EDIT1

The reason for use the attribute window in editor is when I tried to use update feature, it didn't work.

Actally my basic idea is after user create new feature:

(1) use max OBJECTID to identify the new created feature

(2) provided a window to ask user to input the PARCEL_ID and SUB_ID of the new created feature

(3) use feature.store to update the PARCEL_ID and SUB_ID values

But I failed to make it work. My code is like followings:

Private Sub GetMaxFID()
    Try
        Dim pMxDoc As IMxDocument, pFLayer As IFeatureLayer
        Dim pCursor As ICursor
        pMxDoc = My.ArcMap.Application.Document

        Dim pMap As IMap
        Dim pActiveView As IActiveView

        pMap = pMxDoc.FocusMap
        pActiveView = pMxDoc.FocusMap

        Dim layerNum = GetIndexNumberFromLayerName(pActiveView, My.Settings.ParcelLayer)
        pFLayer = pMxDoc.FocusMap.Layer(layerNum)
        pCursor = pFLayer.Search(Nothing, False)

        If TypeOf pCursor Is IFeatureCursor Then
            Dim pData As IDataStatistics = New DataStatisticsClass
            pData.Field = "OBJECTID"
            pData.Cursor = pCursor
            Dim pStatResults As IStatisticsResults = pData.Statistics
            newOBJECTID = pStatResults.Maximum
        End If

    Catch ex As Exception
        logger.Error(ex.ToString)
    End Try
End Sub

Protected Overrides Sub OnClick()
    Try
        Dim pMxDoc As IMxDocument
        Dim pMap As IMap
        Dim pActiveView As IActiveView

        pMxDoc = My.ArcMap.Application.Document
        pMap = pMxDoc.FocusMap
        pActiveView = pMxDoc.FocusMap

        Dim layerNum = GetIndexNumberFromLayerName(pActiveView, My.Settings.ParcelLayer)
        Dim pFLayer As IFeatureLayer

        pFLayer = pMap.Layer(layerNum)

        GetMaxFID()

        Dim featureClass As ESRI.ArcGIS.Geodatabase.IFeatureClass = pFLayer.FeatureClass

        If featureClass Is Nothing Then
            Return
        End If

        Dim feature As IFeature = pFLayer.FeatureClass.GetFeature(newOBJECTID)  ' is this line correct?

        If feature Is Nothing Then
            Return
        End If

        Dim objectIDFieldIndex As Integer = pFLayer.FeatureClass.FindField(newOBJECTID)

        feature.Value(1) = "test123"   'This line I want to set the PARCEL_ID of the new created feature. 
                                       'is it correct to use .value(1) here? what should be the value of (1) in this line? 
        feature.Value(2) = "test123-12" 'This line I want to set the SUB_ID of the new created feature. 
                                       'is it correct to use .value(2) here? what should be the value of (1) in this line? 

        feature.Store()

        'MsgBox("update finished")

    Catch ex As Exception
        logger.Error(ex.ToString)
    End Try
End Sub

I believe there is something wrong with my code, how can I make it work? thanks.