I am using SharePoint 2013 (On-premise) and Nintex Workflow 2013 Enterprise. I have a requirement which needs to provision a user in Exchange Online (create a mailbox for a user). The versions listed does not include Exchange Online and I am not sure what to provide in the "Exchange database" field if I choose Exchange 2013.
i think you can use the web request action and use the graph API to create users. See this MSDN article as a reference: https://msdn.microsoft.com/library/azure/ad/graph/api/users-operations#CreatUser