Hi Rida,
If you go to the page you want to embedd the PowerApp to, and right click on the page you get a menu that where you can Personalize the form, in the bottom of the form, you click the blue field "Personalize this form.

Then you get a new menu where you click insert and PowerApp, from there you choose where you want to embed you PowerApp and follow the steps from there.
Hope this helps!