In order to enable release candidate creation, you need to first choose which penultimate environment in your pipeline to use. If you click the ⚙️ cog icon on your pipeline screen and go into 'Edit pipeline details', this setting can be found within the 'Releases' tab.
You will then be able create a release within your nominated environment, just as you would have done at production.
The selected release candidate environment will show a new Release candidates tab which can be used (in the same way as the Releases tab on production) to add features to release candidates, schedule deployments and abandon a release candidate.
For more information on adding features with releases and release candidate, please refer this article.