This page explains that you may only apply for permission to stay here as the dependant of a post-study worker (Tier 1 Post-study work) if you currently have permission to stay in the United Kingdom as their dependant and they are applying to switch into this category. If you are in the United Kingdom as a dependant and wish to apply as a post-study worker, you must leave the United Kingdom and make an application for entry clearance from overseas. You are unable to switch into becoming the main applicant for this category.