The Free Movement Agreement (FZA, in force since 2002) regulates the residence and employment of EU/EFTA citizens in Switzerland. If you work for more than 90 days, an L (≤ 12 months) or B (> 12 months) permit is required; for < 90 days, the online reporting procedure (easyrelease.admin.ch) is sufficient.
Sex work is covered by the FZA as self-employed employment. Third-country nationals are not covered by the FZA and receive practically no permits for sex work.