@tony , Apologies for the delay!
For App Service, you would purchase the Isolated Stamp for reserved capacity/Isolated Plan type.
Kindly see the step-step process 'Buy Isolated Stamp reserved capacity (check the screenshot 5)- and let me know if this is what you have tried.
The reservation is applied automatically to the number of Isolated stamps that match the reserved capacity scope and region. You don't need to assign a reservation to an isolated stamp. The App Service Isolated Stamp Fee discount is applied to running isolated stamps on an hourly basis.
Kindly let me know, I'll follow-up with you further.