Thanks for your posting. Since TestRail 3.1, you can use the new template system for the defect plugins to customize the default description/comment for a new issue:
The template can be customized in the administration area where you configure the defect integration (e.g. Administration > Integration or on the Integration tab of the respective project under Administration > Projects).
That said, the test URL should already be included in the default description (unless you’ve customized the defect plugin).