You don't need anything to sign up to Taxumo!

Having your Certificate of Registration (aka COR / Form 2303) will help though as this will help you setup your account.

If you're wondering about how to go about business registration --

For a Freelancer, the process is much easier as you usually just have to transact with the BIR. Note that the process does change between RDO's so your mileage may vary.

Here's a guide written by Ginger that details how to register as a freelancer:

Freelancers’ Questions About Taxation, Answered!
​https://mommyginger.com/a-freelancers-and-consultants-guide-to-using-taxumo.html

Now if you're registering your business as a Sole Proprietor, it's a little bit more complicated. Fortunately, Ginger also wrote a guide on how to go through the process:

Legalities in Starting a Business

To know more about Taxumo's BizReg service, feel free to visit this page:

https://www.taxumo.com/business-registration/

Hope these help!