Any block on the internal registration instance pages should be able to add the code below. In our case, we added it to the pre-html of the registration instance - navigation block so that it would appear just above the tabs.
Side note: Every tab on that page is actually a link to a separate page with a different block. For us, we added this code to each of those blocks so that the copy widget would appear across all of them. One advanced option is to save the code in your content folder and then use a lava "include" to include it on each page without having multiple versions of it to upkeep.
Your code may vary from ours, the main point is that you need to capture the link to your public page and insert it into the lava shortcode as the "input".
In the example below, we've chosen to only return the first slug from linkages, or return the query parameter version of the link if a slug hasn't been set.