For a job to be listed on the portal, it must have a postcode within the portal region and be listed on Adzuna. To make sure your job is listed on Adzuna you can either add it directly to the Adzuna platform or add it onto a platform that Adzuna pulls from.
To find out which platforms Adzuna pulls from, do a straightforward search for jobs and view the results. You will see jobs appear from the likes of Total Jobs, Reed, Monster, CV Library etc. Then you can decide which platform is best suited to your business.