You need to create the (modeless) dialog and set the hwnd of the dialog in the code below.
Sub DockableWindow()
Dim oUserInterfaceMgr As UserInterfaceManager
Set oUserInterfaceMgr = ThisApplication.UserInterfaceManager
' Create a new dockable window
Dim oWindow As DockableWindow
Set oWindow = oUserInterfaceMgr.DockableWindows.Add("SampleClientId", "TestWindowInternalName", "Test Window")
' Get the hwnd of the dialog to be added as a child
' CHANGE THIS VALUE!
Dim hwnd As Long
hwnd = 4851096
' Add the dialog as a child to the dockable window
Call oWindow.AddChild(hwnd)
' Don't allow docking to top and bottom
oWindow.DisabledDockingStates = kDockTop + kDockBottom
' Make the window visible
oWindow.Visible = True
End Sub