To create an official WeChat account, just go to apply.wechat.com and fill up the online form. You need to have a copy of your proposal document, official company certificate, company profile and business card of contact person.

apply.wechat.com
apply.wechat.com

The process is free and has no associated registration costs. WeChat has not authorised any agency as the official Official Account (OA) registration partner so all applications has to go through the above mentioned website.

How to create an official WeChat account
How to create an official WeChat account