How pairing the WordPress plugin works
Open the site in SolSEO, choose Connect plugin, and copy the code. It is single use and lasts fifteen minutes. Paste it into the plugin, which exchanges it for a key it keeps and shows to nobody.
From then on the plugin polls SolSEO. SolSEO never opens a connection to your WordPress, so there is nothing to allow through a firewall and no webhook to register.