Hi! I'm facing the same roadblock and wondering whether someone can post a solution... I intend to track the submission of lead forms as conversions.
Support said "You can set up a post-submit redirect on a Form Block to another page on the site and then add the custom code for the Google Ads tracking to that page. However, as was mentioned above I cannot guarantee that any custom code on your site will work as this does fall outside my scope of support. You can review the post-submit options for a Form Block here: https://support.squarespace.com/hc/en-us/articles/206566737-Form-Blocks#toc-add-a-post-submit-message-or-redirect"
Thanks in advance.