Is it possible to submit the invitation letter to get visa?

You can't get the Hk entry permit just with the invitation letter, you also need the following documents:
1. Application for Entry Hong Kong (ID 1003A)
2. one recent photograph affixed on page 1 of the application form ID 1003A
3. your valid passport
4. financial standing, e.g. bank statements, savings accounts passbooks, tax receipts, etc.
5. proof of employment (if any), e.g. company leave letter, salary slips, etc.
6. roun-way air ticket
7. letter issued by your employing company to confirm your proposed business activities in Hong Kong
8. letter issued by a trade body to confirm your attendance at the conference (the invitation letter from the conference may be ok)