4. Receiving a free self-testing kit: Through phone and/or social media interactions, prospective users will provide a mailing address, and the investigators will send them a free HIVST kit (Aware TM Oral) in a plain envelope without any identification of the project. Participants will be given two alternative options for receiving the HIVST kit: 1) the investigators can warp the HIVST kit in a parcel without any identification of the project and send it to a Seven-Eleven store suggested by the participant by express. It is a common practice for people in HK to pick up their express from such stores. Upon receiving the express, a reminder SMS will be sent to the recipient by the store. Participants can pick up the parcel by showing the SMS they received without any additional charge; 2) the kit will be dispatched to them by volunteers. Receiving the test-kit is a strong cue to action (Health Belief Model, HBM) that may induce action. The practice was implemented smoothly in our HMRF project, for which we sent out about 200 self-test kits and most of those receiving the kit took up HIVST-online. The mailing of a free kit also removes logistic barriers required to buy or to collect a self-testing kit and cost concern (HBM). HIV rapid testing now being provided by the local NGOs and the government are also free of charge.