Answer
Open the custom page via Page Configuration's View Page option and look at the resulting URL — it follows a pattern like .../main/ifsapplications/web/page/<Client>/<Page>;path=.... The segment immediately after /page/ in that URL is the value for 'Client' in Navigator Designer, and the following segment (before any ;path= or query parameters) is the value for 'Page'. Set Entry Type to 'PAGE', and Label can be whatever display name you prefer for the entry to show in the navigator.