How to match a URL param to an item in a repeating group

I have a repeating group of posts and using a custom state (text) on the RG I am able to populate it with a post ID based on a URL param.

Is there an way to match the value of the custom state (post ID as text) with the post in the repeating group so that I can extract the index (number) of that particular post? That way I can set that number as the starting number on a list using the toolbox plugin.

Any help with this is immensely appreciated. Thank you!