Check if password box was changed

Im using passwordBox and I want to check if user has type there anything,if yes change button status to enable. how can I check if user type anything in password. It behave differently from text box since you cant bind it to text and when user type anything raise some event.any idea?

I have tried with the following but I got error:

<PasswordBox  Grid.Column="5"
BorderBrush="#FFB0B1AB" ><i:Interaction.Triggers><EventTrigger EventName="KeyDown"><si:InvokeDataCommand Command="{Binding MyCommand}" /></EventTrigger></i:Interaction.Triggers>  </PasswordBox>