How to use the value of a field in a filter criterion? I created a comment view.

The parent content can be flagged and I want to display the comments posted since the date of the flag.

I added a hidden field "Date of creation of the flag".

I added a filter criterion (is greater than or equal to) "Date the comment was posted".

How to use the value of the flag in the filter criterion "Date of publication of the comment" ? there is no token and I have not found a module that can do that.
You could write a custom Views filter plugin to handle your use case, I have found WebOmelette's tutorials invaluable over the years, and he has a great one for D8 Views filters: https://www.webomelette.com/creating-custom-views-filter-drupal-8. This will allow you to fine tune the query to your needs and do any kind of filtering that isn't available out of the box.