When setting Storyboard.TargetName on custom control in trigger, throws exception - wpf

Question

I have custom button control FancyButton (created for reuse obviously) like this below and some code behind (mainly dependency properties):
<Button x:Class="Views.Controls.FancyButton"
...... >
<Button.ContentTemplate>
<DataTemplate>
....
</DataTemplate>
</Button.ContentTemplate>
</Button>
Below, a usage of this control, trigger should start to switch popup property on mouse up but instead of it I'm facing exception that says
'Popup' name cannot be found in the name scope of
'Views.Controls.FancyButton'.
<controls:FancyButton x:Name="ChooseButton" ServerConnection="{Binding CurrentServer}">
<Button.Triggers>
<EventTrigger SourceName="ChooseButton" RoutedEvent="UIElement.PreviewMouseLeftButtonUp">
<BeginStoryboard>
<Storyboard>
<BooleanAnimationUsingKeyFrames Storyboard.TargetName="Popup" Storyboard.TargetProperty="(IsOpen)">
<DiscreteBooleanKeyFrame KeyTime="00:00:00" Value="True"/>
</BooleanAnimationUsingKeyFrames>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Button.Triggers>
</controls:FancyButton >
<Popup Placement="Bottom" Name="Popup">
..........
</Popup>
Previous solution - when used Button control directly - was working as expected but I had a XAML duplication.
What does it change and how to make it work?

Answer 1

Solved!
Instead of using Storyboard.TargetName attached property I used Storyboard.Target and proper binding as below:
<Button.Triggers>
<EventTrigger SourceName="ChooseButton" RoutedEvent="UIElement.PreviewMouseLeftButtonUp">
<BeginStoryboard>
<Storyboard>
<BooleanAnimationUsingKeyFrames Storyboard.Target="{Binding ElementName=popup1}" Storyboard.TargetProperty="(IsOpen)">
<DiscreteBooleanKeyFrame KeyTime="00:00:00" Value="True"/>
</BooleanAnimationUsingKeyFrames>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
Cheers