How to link to GooglePlay to rate a game (Unity3d)?

After clicking on button call

Application.OpenURL ("market://details?id=" + Application.productName);

After a couple of times that the user have used your application, you can popup a message asking the user to rate the app. And if the user tap on Ok on dialogue, you can redirect the him with the below link to playstore for rating.

Link with button pressed "https://play.google.com/store/apps/details?id=" + yourcurrentActivity.this.getPackageName() this link will open google playstore and show your application there.

PS. you can use Sharedprefs to keep record of the times the user launch your application.

As you can change your application name and Google allow different name in the app and in the play store; better use Application.identifier rather than Application.productName;

You can not change your application identifier or Package name after uploading the first APK

public void OnClickRateUs()
{
    Application.OpenURL("market://details?id=" + `Application.identifier`);
}