How to enable custom script for SharePoint Online (Office 365)

Office-365-adminI would like to share my experience with my recent customer in office 365, we manage to register them company in Office 365, but for some reason, after we create new site collection we are not able to add new content editor,  adding custom script and another big problem, we are not allowed to modify the master page in SharePoint Designer 2013, I was confused about all of this problem .
So started to do some research on google and finally found the solution;
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