As a Bangladeshi citizen I like to attend a meeting organized by NGO. I visited china embassy web sit for visa requirement of Hong Kong and necessary document.

You need to prepare application form with recent taken photo, proof of your financial standing & proof of employment if any, your round trip flight tickets for HK, your meeting's schedule if applicable and invitation letter from the meeting.
You need to apply through your local China embassy or mail all the documents to HK Immigration Department at Receipt and Despatch Unit, Hong Kong Immigration Department, 2/F, Immigration Tower, 7 Gloucester Road, Wan Chai, HK.