A few weeks ago, I’ve submitted my first contribution to the community repository of SharePoint Framework solutions, in this case for the extension. This was built with the purpose to send a message to your external users that access your SharePoint site. You can present this message to all your SharePoint sites using the Tenant wide extension list on your Tenant App Catalog.
You can find the solution here – https://github.com/SharePoint/sp-dev-fx-extensions/tree/master/samples/js-guest-message
All the required steps to implement this solution on your SharePoint site are described in the link above. I will share some useful links: