Found endpoint that returns rokuCustomerId on the Web Service API that accepts transaction ID.
Don't see way to get customer data, like Roku ID, without transaction ID.
In addition to Customer ID, you can also collect a user's name and email address at purchase time using the GetUserData method.
It's best to limit this information to name and email, unless you have a specific requirement (e.g. zip code is needed to tell user about events in his/her area). This increases the likelihood of a user signing up on-device.