In my outlook add-in, I notice that the license token is not passed as ?et query string at all. I did some debugging and find out the following: it seems that the value of the StoreType value within OsfControl is always hardcoded to "exchange" in https://r1.res.office365.com/owa/prem/16.1569.8.2186260/scripts/microsoft.owa.extensibilitynext.js which leads to undefined _omnexEntitlment when the etoken is retrieved it's always been retrieved with the initial empty string value.
getEToken: function() {
return this._omexEntitlement ? this._omexEntitlement.etoken : this._etoken
},
add-ins (dropbox to outlook import)