I didn't. Instead, I manually subscribed to hundreds of newsletters myself to get started. Eventually thousands. It wasn't until 4 years in that I added the ability for brands to add themselves. Prior to that, they could only request to be added via an email form.

The thing keeping me sane was a lot of work on backend tools to streamline the process. A ranked queue of brands that I had sourced from various places, and automatically-generated email addresses.