Sci-Hub look up engine for Zotero. Add to zotero engines file (https://www.zotero.org/support/locate#managing_lookup_engines). Afterwards, relaunch Zotero, make sure the newly added lookup engine is checked under Locate (green arrow) -> Manage Lookup Engines.

Zotero-Sci-hub.json

[
    {
        "name": "Sci-Hub Lookup",
        "alias": "Sci-Hub",
        "icon": "null",
        "_urlTemplate": "https://sci-hub.ru/{z:DOI}",
        "description": "Sci-Hub full text PDF search",
        "hidden": false,
        "_urlParams": [],
        "_urlNamespaces": {
            "z": "http://www.zotero.org/namespaces/openSearch#",
            "": "http://a9.com/-/spec/opensearch/1.1/"
        },
        "_iconSourceURI": "http://sci-hub.ru/favicon.ico"
    }
]