What is the best way to encourage university girls to wear the legal hijab, taking into account the prevalence of immodest clothing?

The best way to invite a negligent Muslim is to remind them of their love for Allah, their faith in Him, His Messenger, and His Sharia. Emphasize that this love necessitates obedience, for the lover is obedient to the beloved. Remind them that the hijab is part of Allah's Sharia, and that Islam is in harmony with human nature (fitra) and serves the welfare of creation. Remind them of the purpose of creation, which is to worship Allah by following His Sharia, and that they are tested with religious obligations so that Allah may know who among them is best in deeds, making them worthy of His Paradise. Support this with the two verses: "And I did not create the jinn and mankind except to worship Me" and "He who created death and life to test you—which of you is best in deed."