public event DesignerLoadedEventHandler DesignPanelLoaded

Public Event DesignPanelLoaded As DesignerLoadedEventHandler

using DevExpress.XtraReports.UI;
using DevExpress.XtraReports.UserDesigner;
using System.Drawing.Design;
// ...

private void button1_Click(object sender, System.EventArgs e) {
    // Create a Design Tool with an assigned report instance.
    ReportDesignTool designTool = new ReportDesignTool(new XtraReport1());

    // Access the standard or ribbon-based Designer form.
    // IDesignForm designForm = designTool.DesignForm;
    IDesignForm designForm = designTool.DesignRibbonForm;

    // Handle the Design Panel's Loaded event.
    designForm.DesignMdiController.DesignPanelLoaded += DesignMdiController_DesignPanelLoaded;

    // Load a Report Designer in a dialog window.
    // designTool.ShowDesignerDialog();
    designTool.ShowRibbonDesignerDialog();
}

void DesignMdiController_DesignPanelLoaded(object sender, DesignerLoadedEventArgs e) {
    // Access the Toolbox service.
    IToolboxService toolboxService =
        (IToolboxService)e.DesignerHost.GetService(typeof(IToolboxService));

    // Add a custom control to a new category.
    toolboxService.AddToolboxItem(new ToolboxItem(typeof(XRZipCode)), "New Category");

    // Add a custom control to the default category.
    // toolboxService.AddToolboxItem(new ToolboxItem(typeof(XRZipCode)));
}

Imports DevExpress.XtraReports.UI
Imports DevExpress.XtraReports.UserDesigner
Imports System.Drawing.Design
' ...

Private Sub button1_Click(sender As Object, e As System.EventArgs)
    ' Create a Design Tool with an assigned report instance.
    Dim designTool As New ReportDesignTool(New XtraReport1())

    ' Access the standard or ribbon-based Designer form.
    ' Dim designForm As IDesignForm = designTool.DesignForm
    Dim designForm As IDesignForm = designTool.DesignRibbonForm

    ' Handle the Design Panel's Loaded event.
    AddHandler designForm.DesignMdiController.DesignPanelLoaded, _ 
        AddressOf DesignMdiController_DesignPanelLoaded

    ' Load a Report Designer in a dialog window.
    ' designTool.ShowDesignerDialog()
    designTool.ShowRibbonDesignerDialog()
End Sub

Private Sub DesignMdiController_DesignPanelLoaded(sender As Object, e As DesignerLoadedEventArgs)
    ' Access the Toolbox service.
    Dim toolboxService As IToolboxService = _ 
        DirectCast(e.DesignerHost.GetService(GetType(IToolboxService)), IToolboxService)

    ' Add a custom control to a new category.
    toolboxService.AddToolboxItem(New ToolboxItem(GetType(XRZipCode)), "New Category")

    ' Add a custom control to the default category.
    ' toolboxService.AddToolboxItem(New ToolboxItem(GetType(XRZipCode)))
End Sub

// to override the SaveCommandHandler for every loaded report.
mdiController.DesignPanelLoaded +=
    new DesignerLoadedEventHandler(mdiController_DesignPanelLoaded);

// Handle the DesignPanelLoaded event.
designForm.DesignMdiController.DesignPanelLoaded +=
    DesignMdiController_DesignPanelLoaded;

connectionProviderService);
designer.DesignRibbonForm.DesignMdiController.DesignPanelLoaded +=
    DesignMdiControllerOnDesignPanelLoaded;

typeof(IConnectionStorageService), connectionStorageService);
    this.reportDesigner1.DesignPanelLoaded += DesignMdiControllerOnDesignPanelLoaded;
}

IDesignForm designForm = designTool.DesignForm;
designForm.DesignMdiController.DesignPanelLoaded += DesignMdiController_DesignPanelLoaded;
designTool.ShowDesignerDialog();

' to override the SaveCommandHandler for every loaded report.
AddHandler mdiController.DesignPanelLoaded, AddressOf mdiController_DesignPanelLoaded

' Handle the DesignPanelLoaded event.
AddHandler designForm.DesignMdiController.DesignPanelLoaded, AddressOf DesignMdiController_DesignPanelLoaded
' Load a report into the Designer.

GetType(IConnectionProviderService), connectionProviderService)
AddHandler designer.DesignRibbonForm.DesignMdiController.DesignPanelLoaded,
    AddressOf DesignMdiControllerOnDesignPanelLoaded

ReplaceService(Me.reportDesigner1, GetType(IConnectionStorageService), connectionStorageService)
    AddHandler Me.reportDesigner1.DesignPanelLoaded, AddressOf DesignMdiControllerOnDesignPanelLoaded
End Sub

Dim designForm As IDesignForm = designTool.DesignForm
AddHandler designForm.DesignMdiController.DesignPanelLoaded, AddressOf DesignMdiController_DesignPanelLoaded
designTool.ShowDesigner()