As per my understanding, employer branding and HR branding are the same. Employer branding or HR branding is further divided into two, namely - internal branding and external branding.
Internal branding involves the efforts taken by the company to build the image of the company among its internal customers (employees).
External branding pertains to the efforts taken by the company to build the image of the company among its stakeholders (public, external customers, media, and society as a whole).
So before you start your seminar, determine the scope by choosing internal, external, or both.