This javascript completely owes to @Deleetdk's code. Since the sci-hub website redirected by the original code is no longer usable. I updated it here. Besides, here I try to make the steps clearer, even if you have zero knowledge about javascript and creation of bookmark on a browser.
1️⃣ Hover your mouse over the bookmark bar of your navigator (e.g., Chrome);
2️⃣ Right click in blank area of the bookmark bar;
3️⃣ Find the option "create webpage (G)" (ps. depend on your navigator, naming could be slightly different);
4️⃣ Now you can add your own bookmarklet to the pop-up window;
5️⃣ Name: (let's say) Sci-hub Passkey; Website: copy and paste the javascript;
6️⃣ Voila! You will be seeing a new bookmarklet on your bookmark bar. That means you successfully created your sci-hub passkey.
Easy peasy! You just need to go to the webpage of that paper (say, an article from Elsevier) that you want to download. And in that very page there is usually a DOI. Now, when you click on the newly created bookmarklet, it should be able to automatically identify the DOI and redirect you to PDF download page of Sci-Hub.
I choose a lazy person to do a hard job. Because a lazy person will find an easy way to do it. (Bill Gates)
Why should you use Sci-Hub Passkey instead of the conventional way you do? Well, simply because you can save the time of
✅ Copying DOI
✅ Opening Sci-Hub webpage
✅ Pasting DOI
You may think this only cost you like two or three seconds. But remember that quantity changes lead to quality changes. If you are a frequent user of Sci-Hub, this trick will save you a lot of time.