Hi Marta, The support article that you link references the 'ga' function in the context of analytics.js This is the file used by Google Universal Analytics while ga.js is used by the standard(and premium) Google Analytics - see the link provided above. My suspicion is that you are using the standard GA tag but calling a function that is only present in the GUA tag. If we are on the right path here, I would add the GUA tag, then add a javascript Extension with your tracker.get function, then scope that Extension to the GUA tag. You may need to adjust sequence as you've described in your other efforts(pre-Loader, DOM Ready) to see when/where the funtion needs to fire.
For added help, if you could provide a link to where you are testing(tagmania site?) and the Account / Profile one of our service engineers can take a quick look and offer suggestions.
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