snappymail/plugins/login-o365/LoginOAuth2.js

((rl) => {
  const client_id = rl.pluginSettingsGet("login-o365", "client_id"),
    // https://learn.microsoft.com/en-us/entra/identity-platform/reply-url#query-parameter-support-in-redirect-uris
    tenant = rl.pluginSettingsGet("login-o365", "tenant"),
    login = () => {
      document.location = "https://login.microsoftonline.com/" +
        tenant +
        "/oauth2/v2.0/authorize?" +
        new URLSearchParams({
          response_type: "code",
          client_id: client_id,
          redirect_uri:
            document.location.href.replace(/\/$/, "") + "/LoginO365",
          scope: [
            // Associate personal info
            "openid",
            "offline_access",
            "email",
            "profile",
            // Access IMAP and SMTP through OAUTH
            "https://outlook.office.com/IMAP.AccessAsUser.All",
            "https://outlook.office.com/SMTP.Send",
          ].join(" "),
          state: "o365",
          access_type: "offline_access"
          // prompt: "consent",
        });
    };

  if (client_id) {
    addEventListener("sm-user-login", (e) => {
      const email = (e.detail.get("Email") || "").toLowerCase();
      if (/@(outlook\.com|hotmail\.com|live\.com)$/.test(email)) {
        e.preventDefault();
        login();
      }
    });

    addEventListener("rl-view-model", (e) => {
      if ("Login" === e.detail.viewModelTemplateID) {
        const
		  container = e.detail.viewModelDom.querySelector("#plugin-Login-BottomControlGroup"),
          btn = Element.fromHTML('<button type="button">Outlook</button>'),
          div = Element.fromHTML('<div class="controls"></div>');
        btn.onclick = login;
        div.append(btn);
        container && container.append(div);
      }
    });
  }
})(window.rl);