The fb:app_id meta tag is added automatically to NationBuilder unless you're using a custom domain.

Since you're using a custom domain, you will need to go to the Facebook Developers page and apply for a developer account. Then, go to "Apps" → "Create a new app" to obtain a new App ID for your site. Follow these steps to obtain your fb:admins and/or fb:app_id embeds, and insert them before the closing </head> tag in your layout.html template file.