Aliexpress
Click here… to search for OnePlus 5T 8/128GB Black HK on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong. Coupons for OnePlus 5T 8/128GB Black HK? Nope, there’re no coupons in Banggood.
Link: Banggood product page
Price: -3 USD
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Old coupons (from the oldest to the newest): 128BGOP, 128BGOP5T
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for OnePlus 5T 8/128GB Black HK.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).