You can request a promotional code from http://www.ibm.com/marketplace/cloud/performance-testing/us/en-us. That page also has a link to IBM Cloud Marketplace from where you can request for a subscription ID. When you get a promotional code or subscription ID, you add it to along with the IBM ID.
Trial Promotional code
A trial promotional code is used to evaluate the load generation capability in the cloud for a limited period of time without any cost. However, there are certain restrictions such as the number of virtual users and the number of agents that can be used in the schedule. The expiry date of the promotional code is calculated from the US Pacific time zone. A trial promotional code is subject to the Trial License Terms and Conditions that are accepted at the time of product install. The promotional code that is required to run a schedule in the SoftLayer public cloud infrastructure is different from the one required for the VMware private cloud infrastructure.
Subscription ID
A subscription ID is required to run a schedule without any limitations on the duration and the number of agents. Use portal to view more information and register for the service. On successful registration, a confirmation email is sent to you. An IBM ID can be associated with multiple subscription IDs. Similarly, in a team environment, multiple IBM IDs can share the same subscription ID. The IBM ID that is associated with the purchase of the subscription can add additional subscribers.