I think you are looking for a "postMessage" from the redirect_uri from your authWindow.. in the semrush code you have pointed to thats /oauth2/umbraco/success https://github.com/umbraco/Umbraco.Cms.Integrations/blob/main/src/Umbraco.Cms.Integrations.SEO.Semrush/Configuration/SemrushSettings.cs#L19-L20 I also found a blog post from @andybutland0280 that seems to relate to the hubspot integration

if (window.opener && typeof opener.postMessage === 'function') {
  opener.postMessage({
    type: 'hubspot:oauth:success',
    url: location.href,
    code: '@Model.AuthorizationCode'
  }, '*');
  window.close();
}

presumably is what goes on when the /oath2/umbraco/success callback url is hit.