How to update emoji in vader lexicon?

I want to update the chart up and chart down emojis sentiment values in the vader lexicon. Seeing this post (VaderSentiment: unable to update emoji sentiment score) I have tried to replicate it but with no success:

new_words = {
    "chart decreasing" : -1,
    "📉" : -1
}
analyzer.lexicon.update(new_words)

analyzer.polarity_scores("📉")

Result: {'neg': 0.0, 'neu': 1.0, 'pos': 0.0, 'compound': 0.0}

It should show neg: 1.0, as I have just updated the lexicon.

Any ideas how to update the values?

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