You have to use IsOpen
property to open the dialog.
I am pasting here the working code for me.
XAML
<Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
<Button Content="Show Dialog" Click="btnShowDialog_Click" />
<callisto:CustomDialog
x:FieldModifier="public"
x:Name="suggestEventDialog"
Title="Suggest an Event"
Background="Teal"
BackButtonVisibility="Visible"
BackButtonClicked="suggestEventDialog_BackButtonClicked_1">
<StackPanel>
<TextBlock
Margin="0,0,0,8"
Text="Suggest an event that should be added to Heron."
FontSize="14.6667"
FontWeight="SemiLight"
TextWrapping="Wrap" />
<TextBlock
Margin="0,0,0,8"
FontSize="14.6667"
FontWeight="SemiLight"
Text="Event URL" />
<callisto:WatermarkTextBox
HorizontalAlignment="Left"
Watermark="http://www.example.com"
Width="400"
Height="35" />
<StackPanel
Margin="0,20,0,0"
HorizontalAlignment="Right"
Orientation="Horizontal">
<Button Content="OK" Width="90" Margin="0,0,20,0" />
<Button Content="CANCEL" Width="90" />
</StackPanel>
</StackPanel>
</callisto:CustomDialog>
</Grid>
C#
private void btnShowDialog_Click(object sender, RoutedEventArgs e)
{
suggestEventDialog.IsOpen = true;
}
private void suggestEventDialog_BackButtonClicked_1(object sender, RoutedEventArgs e)
{
suggestEventDialog.IsOpen = false;
}