Is there any kind of certificate or legal form in Russia for working for a foreign software customer?

Question

Freelance

The customer requires to send a Legal form or a Certificate allowing work with foreign companies, which supposedly I may have. What to send them?
In general, I work with them through Upwork, respectively, the hours are beat there, and so on.

Answer 1

IP registration certificate?

Answer 2

If you work through Upwork, then accordingly, the customer does not work with you personally, but with Upwork Global Inc. Advise the customer with legal and financial issues to contact their technical support.

Answer 3

There are no such things in Russia