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Yes I’m using Gmail and when I use an add on in Safari it asks me to add it to trusted pages
Are you trying to add some sort of Safari extension? Some of those have a popup that warns the extension can see your data and asks if you want to "Trust" the extension. Is that what you are seeing?
 
I was using Google Chrome but when I switched to Safari I got that message
If you are using Safari with a GMail extension that you installed form Google and you know it is safe, then it is okay to click and accept that "Trust" box.
 
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