When you open an iframe inside of Trello via t.popup, t.modal, and t.boardBar, we implicitly sign the URL for you using t.signUrl. Additionally, attachment-section and card-back-section iframes are required to be explicitly signed by you. Signing a URL for an iframe is what allows you to use the client library on the newly opened iframe via the t. methods.
What we mean by “signing the url” is adding context specific to your Power-Up to the url, so that when the secondary iframe is opened, it has the context it needs in order to communicate with Trello. This context is added as the hash for the url (the part that comes after the `