There are two options for you,
1. Apply for Hk visit visa from China embassy in Tehran.
2. Send the HK visit visa directly to HKSAR Immigration Department either directly by post or through a local sponsor

If your visit in Hong Kong is less than 14 days, the China embassy in Tehran can issue the HK visa in 4 working days, otherwise, the embassy should get the approval from the immigration department in Hk, it takes about 1 month.

if you send the application derectly to the immigration department, it takes about 4 weeks.