I want to know the visa requirements after getting the invitation letter from Hongkong

What is your purpose of visiting HK? You should apply for a proper one according to your visiting purpose. There is no HK embassy in your country. Previously, Chinese embassy in your country can deal with your application. But I don't know whether they still handle HK entry permit things. You'd better contact the embassy to confirm this. By the way, you should ask whether you have to submit your application in person.

According to what I know, those people who apply Chinese entry permits are required to make an appointment first and submit their application on their own because they need to do an interview.