How to format DateTime Field in C# WPF application as yyyy-MM-dd?

I am building a C# app to generate codes.  To that end I have created a XAML page to collect the information about the codes to be created.  This page contains two DatePicker fields to indicate when the begin and end dates as to when the code is valid.  However, I need to write this data into a MySQL database, so I need the date to be formatted as yyyy-MM-dd (i.e. 2020-03-10).  After much searching online, I am unable to figure this out.

Here is the code that I have thus far:
GiftCodeGenerator.xaml

<Page x:Class="WSP_USA_Admin_Dashboard.Pages.GiftCodeGenerator"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:WSP_USA_Admin_Dashboard.Pages"
      mc:Ignorable="d" 
       Height="Auto" Width="600"
      Title="GiftCodeGenerator">
    <Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="20*"/>
            <ColumnDefinition Width="25*"/>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="20*"/>
            <ColumnDefinition Width="25*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="50"/>
            <RowDefinition Height="50"/>
            <RowDefinition Height="50"/>
        </Grid.RowDefinitions>
        <TextBlock Grid.Row="0" Grid.ColumnSpan="5" TextWrapping="Wrap" Padding="10">
            Use the below form to generate new gift codes for use on the WSP USA Gift Store.  The Begin and End dates are optional and indicate the dates that the code is valid.  If you leave the begin date empty, it will be valid immediately.  If you leave the end date empty, it will never expire.
        </TextBlock>
        <Label Grid.Column="0" Grid.Row="1" Content="Number of codes*" VerticalAlignment="Center" Margin="5" />
        <TextBox x:Name="CodeQty" Grid.Column="1" Grid.Row="1" Height="25" Margin="5" Width="100" VerticalContentAlignment="Center" />
        <Label Grid.Column="0" Grid.Row="2" Content="PO Number*" VerticalAlignment="Center" Margin="5" />
        <TextBox x:Name="PONumber" Grid.Column="1" Grid.Row="2" Height="25" Margin="5" Width="100" VerticalContentAlignment="Center" />
        <Rectangle Grid.Column="2" Grid.Row="1" Grid.RowSpan="2" VerticalAlignment="Stretch" Fill="LightSlateGray" Width="1" />
        <Label Grid.Column="3" Grid.Row="1" Content="Begin Date" VerticalAlignment="Center" Margin="5" />
        <DatePicker x:Name="BeginDate" Grid.Column="4" Grid.Row="1" Margin="5" Width="100" Height="25" HorizontalAlignment="Left" VerticalAlignment="Center" />
        <Label Grid.Column="3" Grid.Row="2" Content="End Date" VerticalAlignment="Center" Margin="5" />
        <DatePicker x:Name="EndDate" Grid.Column="4" Grid.Row="2" Margin="5" Width="100" Height="25" HorizontalAlignment="Left" VerticalAlignment="Center" />
        <Button x:Name="GenerateCodes" Content="Generate Gift Codes" Grid.ColumnSpan="5" Grid.Row="3" Height="Auto" HorizontalAlignment="Center" Padding="5" Background="DarkRed" Foreground="White" Width="Auto" VerticalAlignment="Center" VerticalContentAlignment="Center" Click="GenerateCodes_Click" />
    </Grid>
</Page>

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GiftCodeGenerator.xaml.cs

    private void GenerateCodes_Click(object sender, RoutedEventArgs e)
    {
      if (CodeQty.Text == "" || PONumber.Text == "")
      {
        MessageBox.Show("The quantity and PO Number fields are required!");
        return;
      }
      else
      {
        int numCodes = Convert.ToInt32(CodeQty.Text);
        string poNumber = PONumber.Text;
        string ValidFrom = BeginDate.Text;
        string ValidTo = EndDate.Text;
        MessageBox.Show("The values entered include:\n# codes: " + numCodes.ToString() + "\nPO #: " + poNumber + "\nValid From: " + ValidFrom + "\nValid To: " + ValidTo);
      }
    }

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Currently, the date looks like "03/10/2020".  I have tried the following example I found online, but got an error about no method of "Value".

var datestring = datePicker.Value.ToString("dd/MM/yyyy", CultureInfo.InvariantCulture);

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