Fill WinForms Control to Parent

I am attempting to build a simple text editor in F# with WinForms. I would like to know how to get items to fill their parent containers most easily, and I am finding that a text box is not filling its parent despite its Dock property being set to DockStyle.Fill. I would like both the top MenuStrip and the RichTextBox to fill the parent containers and take up the whole form (as you'd expect to see in a text editor). Here is a screenshot demonstrating the issue:

As you can see, it would be nice to have both the MenuStrip and RichTextBox to fill the entire form.

Here is the code (along with the .fsproj file):

Program.fs:

module FsEdit.Program

open System
open System.Windows.Forms

[<EntryPoint; STAThread>]
let main argv =
    Application.Run FsEdit.MainForm.MainForm
    0

MainForm.fs:

[<RequireQualifiedAccess>]
module FsEdit.MainForm

open System.Windows.Forms

// MenuBar
let private FileMenuTab =
    new ToolStripMenuItem(
        Text = "File"
    )

let private EditMenuTab =
    new ToolStripMenuItem(
        Text = "Edit"
    )

let private AboutMenuTab =
    new ToolStripMenuItem(
        Text = "About"
    )

let private MainMenuStrip =
    new MenuStrip(
        Text = "MainMenuStrip"
    )

let private allMenuStripItems =
    [|
        FileMenuTab
        EditMenuTab
        AboutMenuTab
    |]
    |> Array.map (fun tab -> tab :> ToolStripItem)
    
MainMenuStrip.Items.AddRange(allMenuStripItems)

// Text Editor
let private MainTextBox =
    let richTextBox = new RichTextBox(
        Text = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut a eleifend nunc. Suspendisse non purus varius, ullamcorper arcu et, vehicula lacus. Integer pellentesque facilisis interdum. Aliquam id leo arcu. Nam mauris nisl, semper eget massa sed, aliquam convallis lacus. Etiam a neque blandit, sollicitudin nisl quis, ornare dui. Aliquam nec lorem sit amet arcu iaculis elementum rutrum eu velit. Curabitur dignissim blandit ligula at efficitur. Curabitur id justo quis tortor egestas ultrices. Nam arcu quam, ullamcorper id velit quis, aliquam finibus libero. Pellentesque semper fermentum sem a scelerisque. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia curae;"
    )
    richTextBox.Dock <- DockStyle.Fill
    richTextBox.Anchor <- AnchorStyles.Left ||| AnchorStyles.Right ||| AnchorStyles.Top
    richTextBox.AllowDrop <- true
    richTextBox.AutoSize <- true
    richTextBox

let private MainTextBoxPanel =
    let p = new FlowLayoutPanel()
    p.Dock <- DockStyle.Fill
    p.WrapContents <- false
    p.FlowDirection <- FlowDirection.TopDown
    p.Anchor <- AnchorStyles.Left ||| AnchorStyles.Right ||| AnchorStyles.Top
    p

MainTextBoxPanel.Controls.Add(MainMenuStrip)
MainTextBoxPanel.Controls.Add(MainTextBox)

// MainForm
let MainForm =
    let form = new Form(
        Text = "FsEdit"
    )
    form.Width <- 800
    form.Height <- 600
    form
    
MainForm.Controls.Add(MainTextBoxPanel)

FsEdit.fsproj:

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <OutputType>WinExe</OutputType>
        <TargetFramework>net7.0-windows</TargetFramework>
        <UseWindowsForms>true</UseWindowsForms>
        <UseWpf>true</UseWpf>
    </PropertyGroup>

    <ItemGroup>
        <Compile Include="MainForm.fs" />
        <Compile Include="Program.fs" />
    </ItemGroup>

</Project>

How do I get these Control objects to fill their respective parents successfully?