Hi @ngdnam88. This is not an area I focus on, so the links below, and quickie code example are just for general reference and guidance, and shouldn't be used 'as is', without quite a bit of customization.
Application.CommandManager
CommandManager
CommandManager.ControlDefinitions
ControlDefinitions
ControlDefinitions.AddComboBoxDefinition
ComboBoxDefinition
ComboBoxDefinition.AddItem
Application.UserInterfaceManager
UserInterfaceManager
UserInterfaceManager.ActiveEnvironment
Environment
Environment.Ribbon
Ribbon
Ribbon.QuickAccessControls
CommandControls
CommandControls.AddComboBox
Dim sDispName As String = "MyComboBox1_DisplayName"
Dim sIntName As String = "MyComboBox1_InternalName"
Dim eClassification As CommandTypesEnum = CommandTypesEnum.kQueryOnlyCmdType
Dim iDropDownWidth As Integer = 25
Dim oCDs As ControlDefinitions = ThisApplication.CommandManager.ControlDefinitions
Dim oComboBoxDef As ComboBoxDefinition = oCDs.AddComboBoxDefinition( _
sDispName, sIntName, eClassification, iDropDownWidth)
oComboBoxDef.AddItem("ComboBoxItem #1")
oComboBoxDef.AddItem("ComboBoxItem #2")
Dim oQuickAccessControls As CommandControls = ThisApplication.UserInterfaceManager.ActiveEnvironment.Ribbon.QuickAccessControls
Dim oCmdCntrl As CommandControl = oQuickAccessControls.AddComboBox(oComboBoxDef)
Wesley Crihfield

(Not an Autodesk Employee)