According to a message I got from an community on Fandom, there's a way to bring back the "Add Video" button here.
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"use strict";
if (window.addVideoPatch || mw.config.get("skin") !== "fandomdesktop"
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window.addVideoPatch = true;
var button_text;
function addVideoPatch() {
mw.hook("wikipage.content").remove(addVideoPatch);
var button = document.createElement("A");
button.classList.add("wds-button");
button.classList.add("addVideo");
button.href = "#";
button.textContent = button_text;
button.addEventListener("click", function () {
document.getElementsByClassName("special-videos__upload")[0]
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});
document.getElementsByClassName("page-header__actions")[0].appendChild(button);
}
mw.loader.using("mediawiki.api").then(function () {
(new mw.Api()).getMessages("videos-add-video").then(function (msg_data) {
button_text = msg_data["videos-add-video"];
mw.hook("wikipage.content").add(addVideoPatch);
});
});
})();