From the Admin On Demand FAQs (https://www.splunk.com/pdfs/legal/Splunk-Admin-On-Demand-Services-FAQs.pdf):
To request an Admin on Demand service, use the existing Support Portal. If you have an Admin on Demand entitlement, you will have access to the ability to raise an Admin on Demand case type. This will have a similar interface to the existing support case system and allow you to select the desired service from a list of options.
If this reply helps you, Karma would be appreciated.