How do I embed a URL to a module using my custom resource selection LTI?

Community Novice

So I have an Express LTI tool that I'm developing which returns specific course URLs, but I don't know how to auto-insert those URLs into the "External Tool" tab. I have looked into Khan Academy's LTI and it seems like they return an <iframe> whenever I click on the "embed" button, so the LMS can auto-insert it into the input field. But I have no idea how to do it on my custom LTI, am I supposed to make a POST request or something? And if it's true, what body should I give it?