Hello Kirsten,
We currently do not support modal dialogs that trigger from an onLoad event and you are correct that we block javascript from our text cards. We do this to mitigate the security risk of having malicious code injected and also because people used to load in duplicative instances of jQuery or Bootstrap.js that would collide with our files and cause things like dropdown menus not to work correctly.
We do have a feature for this on our backlog, but I cannot give you an answer for when it'll be included in the product. I, will, however add your request to the issue. In the meantime, is it possible to present that information in an info alert at the top of the page? If you were to use a custom header, you could even position it right near the top. I see a number of alerts around cookies these days that do not require explicit consent and are worded such that if I continue to use the page, I am agreeing to have cookies track my movement.
