Show loading message or a spinner when contents are loading in e-singlive iframe

In our web application we are using an iframe for rendering e-signlive contents for document signing. We chose iframe as part of the solution in order to keep the user within our application. Before bringing iframe up, we show a spinner (loading indicator) while we prepare and upload the documents (from server-side) to e-signlive in the background. After uploading the documents we replace the spinner with iframe (pointing it to the signing url), however when we show the iframe there is a noticeable delay in loading (from e-signlive) the documents/contents inside the iframe. We are trying to improve the user experience by providing some visual indication when documents/contents are loading within the iframe. We are looking for a sample code to either show an spinner or a loading message within the iframe and be able to remove the spinner or message when iframe contents are fully loaded, any help will be highly appreciated!