The first time a SolarNode boots up, you'll end up with a screen asking for an invitation code from SolarNet:

SolarNode devices join SolarNetwork by first generating an invitation on SolarNet and then pasting that invitation into the SolarNode's initial setup screen.

Visit https://data.solarnetwork.net/solaruser/ and use the Invite New SolarNode button under My Nodes to generate the invitation. If you don't have an account on SolarNet yet, register for one. The generated invitation will like this:

Simply copy/paste the invitation into the SolarNode's setup screen and follow the prompts.