How will I apply work visa for Hongkong from Pakistan?

You should ask you employer to transfer the following application materials to the HK Immigration Department for you:
Application form (ID 1001A)
Recent photograph (affixed on page 1 of the application form ID 1001A)
Photocopy of your passport
Photocopy of proof of academic qualifications and relevant working experience
Photocopy of your medical report

In addition, your employer itself should submit:
Application form (ID 1001B)
Four original copies of the standard employment contract (LD 294) signed between the employer and you
Photocopy of the approval-in-principle letter issued by the Labour Department