Hi @lorenmn, welcome back to the Community!
Just as @Spark Bridge Digital LLC mentioned, the Form ID can be found in the URL of your browser while you are editing the form in the following format: https://klaviyo.com/forms/xxxxxx.
Additionally, another common reason why an embedded form might not be appearing on your site is that unlike popup and flyout forms, embed forms require the extra step of manual code insertion before they are visible in the storefront. This Help Center Article details the steps for embedding a form for each integration type (Shopify, BigCommerce, etc.).
In short, while the filter might very well need fixing, the form will not appear unless the above steps are completed. Here’s another active Community thread regarding a similar issue, if you’re interested :)
Another reason why the form is not appearing in your drop-down menu when creating your filter is that no one has filled out the form yet. This type of behavior is similar to a topic my colleague has described in the following Community post:
I’m looking forward to your response and thank you for participating in the Community!
-Brian