Add Reference UnityEngine.Advertisements to Visual Studio 2017

Hi, I have a problem that is my Unity WON’T add automatically reference to UnityEngine.Advertisements. Moreover even I can’t add this reference, because there’s just no ‘Add reference…’ option. But the thing is I do have ‘Add reference…’ option in my other projects but not in this one and even there Unity automatically adds reference to UnityEngine.Advertisements. I think it’s because this Unity project is from older version (not so very old) but I now have installed 2017.2.0f3 and 2017.1.0p5 and move my project to the lates version. I’m using Visual Studio 2017. So how can I fix this or at least add the reference? Thanks!

Dang it… I solved my problem by hitting Project → “project_name” Properties… Something happened (I think Visual Studio automatically readjusted my project) and then I added the reference).