i have to apply hongkong visa thru hk immigration by post or handover to china embassy in srilanka.?

HKTDC is right. You should apply from the Chinese embassy in your country or the HK Immigration Department. No matter where you apply, you should submit your original passport and a copy of your passport information page. I think you can contact the embassy and ask how long your application will be processed. If it is handled by the embassy, you just need to wait for four days. If it is sent to the HK Immigration Department, you will wait for around a month.