Clinical trials are best cited using the Website reference type. You can use the browser extension to import from the URL and tweak the metadata, or create a new manual reference. As a starting point, input the metadata in the following fields:
Author: Author names, or if it's a group, place this in the Author surname field.
Title: Title of trial.
Website Title: Registry name. Identifier. (eg ClinicalTrials.gov. Identifier: NCT123456.)
Publication Date: Date trial was posted/updated
URL: Link to trial
Access Date: Populated automatically but change as needed.
This may need some adjusting to generate an accurate citation depending on your requirements.