I cannot display a popup from a view model

I want to display a community toolkit popup from a view model. The type of popup I want is a message box where I can pass a string variable that is the message I want displayed. I followed this Learn Popup Article from Microsoft, and I was able to build my popup and data bind the popup to a view model without any trouble. My problem comes when I try to display the popup from a different view model. Here is what I have so far:

Popup XAML

<ctm:Popup xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
           xmlns:ctm="http://schemas.microsoft.com/dotnet/2022/maui/toolkit"
           xmlns:vwmod="clr-namespace:LockAndKeyMaui.ViewModels"
           xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
           x:Class="LockAndKeyMaui.MsgBox"
           x:DataType="vwmod:MsgViewModel"
           CanBeDismissedByTappingOutsideOfPopup="False">
    
    <VerticalStackLayout
        WidthRequest="300" HeightRequest="200"
        BackgroundColor="Blue">
        <Label 
            Text="{Binding Msg}"
            TextColor="Yellow"
            VerticalOptions="Center" 
            HorizontalOptions="Center" />
    </VerticalStackLayout>
</ctm:Popup>

Popup C#

using CommunityToolkit.Maui.Views;
using LockAndKeyMaui.ViewModels;

namespace LockAndKeyMaui;

public partial class MsgBox : Popup
{
    readonly string MsgTxt;
    public MsgBox(MsgViewModel msgvw)
    {
        InitializeComponent();
        BindingContext = msgvw;
    }
}

Popup View Model

using CommunityToolkit.Maui.Core;
using System.ComponentModel;

namespace LockAndKeyMaui.ViewModels
{
    public class MsgViewModel : INotifyPropertyChanged
    {
        private string? msg;

        readonly IPopupService popupService;

        public string Msg
        {
            get => msg!;
            set
            {
                msg = value;
                OnPropChg(nameof(Msg));
            }
        }

        public MsgViewModel(IPopupService popupService)
        {
            this.popupService = popupService;
        }

        public event PropertyChangedEventHandler? PropertyChanged;

        private void OnPropChg(string prName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(prName));
        }
    }
}

Now in a new view model I'm trying to use the command that the article shows which is this.popupService.ShowPopup<UpdatingPopupViewModel>(onPresenting: viewModel => viewModel.Name = "Shaun"); but I'm just trying to replace UpdatingPopupViewModel with UpdatingMsgViewModel and that's one of the red underlines I get. I replaced the line with this.popupService.ShowPopup<MsgViewModel>(onPresenting: viewModel => viewModel.Msg = "This is a test."); and that's where the variable Msg does not exist.

I have the popup registered with builder.Services.AddTransient<MsgBox, MsgViewModel>(); in the MauiProgram.cs file.

So what am I missing? Thanks for the help.