Did you already have a Nintex Workflow Cloud tenant? You don't have such a tentant just by being a partner. You can create a tenant and then talk to your local partner manager.
If you already have a tenant and just cannot remember the URL - then your partner manager should be able to help you as well.
Hi, In the Partner Portal navigate to Workflow Platform from the ribbon/top menu.
Under the Nintex Workflow Menu you will see Nintex Workflow Automate processes with clicks, not code. PLATFORM: SharePoint | Office 365 | Nintex Workflow Cloud | Salesforce
Click the Nintex Workflow Cloud link.
On the next page about half way down on the right hand side you will see something like: