Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        FpSpread1.ActiveSheet.Columns(0).CellType = New CustomCellType(FpSpread1)
    End Sub

    Public Class CustomCellType
        Inherits FarPoint.Win.Spread.CellType.GeneralCellType

        Private _spread As FarPoint.Win.Spread.FpSpread

        Public Sub New(ByVal spread As FarPoint.Win.Spread.FpSpread)
            _spread = spread
        End Sub

        Private Function GetButtonWidth(ByVal g As Graphics, ByVal zoomFactor As Single) As Integer
            Return CInt((SystemInformation.VerticalScrollBarWidth * zoomFactor * 96 / g.DpiX))
        End Function

        Public Overrides Sub PaintCell(ByVal g As Graphics, ByVal r As Rectangle, ByVal appearance As FarPoint.Win.Spread.Appearance, ByVal value As Object, ByVal isSelected As Boolean, ByVal isLocked As Boolean, ByVal zoomFactor As Single)
            Dim buttonWidth As Integer = GetButtonWidth(g, zoomFactor)
            g.FillRectangle(Brushes.LightGray, r.Right - buttonWidth, r.Y, buttonWidth, r.Height)
            Dim buttonRect As Rectangle = New Rectangle(r.Right - buttonWidth, r.Y, buttonWidth, r.Height)
            Dim element = System.Windows.Forms.VisualStyles.VisualStyleElement.Button.PushButton.[Default]

            If System.Windows.Forms.VisualStyles.VisualStyleRenderer.IsElementDefined(element) Then
                Dim renderer As System.Windows.Forms.VisualStyles.VisualStyleRenderer = New System.Windows.Forms.VisualStyles.VisualStyleRenderer(element)
                Dim clipBounds As Rectangle = Rectangle.Round(g.ClipBounds)
                renderer.DrawBackground(g, buttonRect, clipBounds)
            Else
                ControlPaint.DrawButton(g, buttonRect, ButtonState.Normal)
            End If

            r.Width -= buttonWidth
            MyBase.PaintCell(g, r, appearance, value, isSelected, isLocked, zoomFactor)
        End Sub

        Public Overrides Function IsReservedLocation(ByVal g As Graphics, ByVal x As Integer, ByVal y As Integer, ByVal rc As Rectangle, ByVal appearance As FarPoint.Win.Spread.Appearance, ByVal value As Object, ByVal zoomFactor As Single) As Object
            If rc.Contains(x, y) AndAlso x >= rc.Right - GetButtonWidth(g, zoomFactor) Then
                Return Me
            End If

            Return MyBase.IsReservedLocation(g, x, y, rc, appearance, value, zoomFactor)
        End Function

        Public Overrides Sub StartEditing(ByVal e As EventArgs, ByVal selectAll As Boolean, ByVal autoClipboard As Boolean)
            Dim mouseEvent As MouseEventArgs = e

            If TypeOf e Is MouseEventArgs And CType(e, MouseEventArgs).Clicks = 1 Then
                Dim result = MessageBox.Show("Click OK to set ""Test"" to cell.", "", MessageBoxButtons.OKCancel)

                If result = DialogResult.OK Then
                    _spread.CancelCellEditing()
                    _spread.ActiveSheet.ActiveCell.Value = "Test"
                End If

                Return
            End If

            MyBase.StartEditing(e, selectAll, autoClipboard)
        End Sub

    End Class

End Class