using DevExpress.Utils;
using DevExpress.XtraEditors;
using DevExpress.XtraEditors.ViewInfo;
using System;
using System.Data;
using System.Windows.Forms;

namespace DXApplication10 {
    public partial class Form1 : DevExpress.XtraEditors.XtraForm {
        public Form1() {
            InitializeComponent();
            // ...
            this.Load += Form1_Load;
        }

        private void Form1_Load(object sender, EventArgs e) {
            CreatePopupEditor();
        }

        void CreatePopupEditor() {
            RichTextBox rtb = new RichTextBox();
            rtb.Dock = DockStyle.Fill;
            PopupContainerControl popupControl = new PopupContainerControl();
            popupControl.Controls.Add(rtb);

            PopupContainerEdit editor = new PopupContainerEdit();
            editor.Location = new Point(100, 30);
            editor.Width = 350;
            editor.Properties.PopupControl = popupControl;
            Controls.Add(editor);

            editor.Closed += (s, a) => {
                (s as PopupContainerEdit).EditValue = rtb.Text;
            };

            editor.CustomDisplayText += (s, x) => {
                if (x.Value != null)
                    x.DisplayText = x.Value.ToString().TrimStart(' ', '\r', '\n');
            };
        }
    }
}

Imports Microsoft.VisualBasic
Imports DevExpress.Utils
Imports DevExpress.XtraEditors
Imports DevExpress.XtraEditors.ViewInfo
Imports System
Imports System.Data
Imports System.Windows.Forms

Namespace DXApplication10
    Partial Public Class Form1
        Inherits DevExpress.XtraEditors.XtraForm

        Public Sub New()
            InitializeComponent()
            ' ...
            AddHandler Me.Load, AddressOf Form1_Load
        End Sub

        Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs)
            CreatePopupEditor()
        End Sub

        Private Sub CreatePopupEditor()
            Dim rtb As New RichTextBox()
            rtb.Dock = DockStyle.Fill
            Dim popupControl As New PopupContainerControl()
            popupControl.Controls.Add(rtb)

            Dim editor As New PopupContainerEdit()
            editor.Location = New Point(100, 30)
            editor.Width = 350
            editor.Properties.PopupControl = popupControl
            Controls.Add(editor)

            AddHandler editor.Closed, Sub(s, a) TryCast(s, PopupContainerEdit).EditValue = rtb.Text

            AddHandler editor.CustomDisplayText, Sub(s, x)
                If x.Value IsNot Nothing Then
                    x.DisplayText = x.Value.ToString().TrimStart(" "c, ControlChars.Cr, ControlChars.Lf)
                End If
            End Sub
        End Sub
    End Class
End Namespace