Get error when add reference to shared code project.

Hello,
I,m new in the Xamarin.Forms. I created a new Xamarin.Forms app for iOS and Android and my app is successful.
Now I like to add windows phone as well but I got following error when I add portable UI project reference to windows phone.

" Unable to add a reference to project 'XamarinFormsApp'. The current project's
target is not one of or compitable with the targets of portable Liabrary project
'XamarinFormsApp'."
A Portable Liabrary projects targets can be changed via the Liabrary tab in project's
properties.

I tried to find Liabrary tab in properties but no luck.

So my question is how to solve above error ??