Currently adding custom AI prompts does not support dynamic placeholders like the URL. This is a huge miss because tab title and highlighted text is often insufficient for a good summary. Also, all chatbots now support web search so sending the URL as context would drastically reduce the window.
So URL should either be hardcoded to be sent with current prompts OR we should be able to add it into existing prompts. For example, here is a custom prompt I added:
user_pref("browser.ml.chat.prompts.5", '{"label": "Summarize with URL", "id": "summarize_with_url", "value": "Summarize the following page.\nURL: {{url}}"}');