This redirect URI should be both entered in Easymorph WebLocation connector and registered with Smartrecruiters during application registration step. The value of redirect_uri passed to SmartRecruiter must exactly match the values you entered in the DataConnector window, including upper/lowercase, terminating slashes, etc.
Probably you have to change redirect_uri setting in SmartRecruiter to match one in Easymorph. Looks like you have to send a corresponding request to the Partner Operations Consultant you have started working with to do this.
I contacted the partners. I was advised that I do not need to register an application as I am developing it for one client. They indicated I only need my client_id and client_secret and that I do not need a redirect URL.