This can be done via IIS Express. However, an HTTPS endpoint must be configured as IdentityServer will redirect to an HTTPS URL if it is not running on a secure connection.
Open the IdentityServer solution in Visual Studio
Select the WebSite project
Press F4 to open the Properties panel
Change the SSL Enabled property to True
The SSL URL property is set with an HTTPS URL running on a different port
Copy the SSL URL value
Open the Properties of the project
On the Web tab, set the Server to IIS Express and the Project URL to the SSL URL value
Save the project and press F5 to debug
On the first run you will get a browser error about the untrusted certificate, click Continue
Update the Relying Party application with the SSL URL as the STS endpoint