There have many been restrictions involved while working on OPT and STEM extension OPT.According to the USCIS, students are not supposed to work through the third party client sites and must work onsite at the employer’s workplace in order to have the STEM extension.
More information in the link below
https://www.shrm.org/resourcesandtools/hr-topics/talent-acquisition/pages/uscis-restricts-third-party-placement-stem-opt-students.aspx
