Click on Customization, then on the Subplatforms tab. Enter the initial domain you want to use (mycompany.skyprepapp.com) and click on Add Subplatform.
You may see a page prompting you to switch plans. If you do, just remove the portion of the URL after the .com portion.
If a user is an Admin in both the parent platform and a subplatform, they will be able to sign into the subplatform by clicking on it in the Single Sign-On section on the Dashboard. However, this will not work if trying to navigate from a subplatform to the parent platform.