Hi Nick. No, that's not possible; but good news is that it doesn't need to be in order to do what you are looking for. If you add the dashboard as a custom tab to your Portfolio layout, the Portfolio ID prompt is redundant and can (should) be removed altogether. When it's run as a custom tab of an object, all records are filtered to the object being viewed. William English T-Mobile
If you like my content, please take a moment to view and vote on my Idea Requests: https://tinyurl.com/4rbpr7hf