Yes, it's a JS-heavy website with a webdesign that makes my eyes hurt. Sigh, can't have nice things, can we?
You can skip right to the signing by going to the relevant Citizens' Initiatives website on europa.eu here:
https://eci.ec.europa.eu/038/public/
Both sites seem to work fine in a Tor Browser, and the signing requires solving a self-hosted captcha (no Big Tech captcha involved, it seems), so at least that's good.
The main method is to look for a verified website link, and in some cases the server addresss of the account will also verify identity.
For example, if you look at @thunderbird you can see it's the official account because it has a verified link to Thunderbird's official website.
Another example, the European Commission's official account @EU_Commission is genuine because its account address shows its server is at europa.eu, which is the official European Union website.
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