what to do if no functional specifications code example

Example 1: testing without requirements

I would talk to client, po, developer and 
try to gain as much requirement as possible
and try to understand the logic of the feature.
Also I would do exploratory testing.

Example 2: what to do if no functional specifications

When there are no functional specifications
or system development documents, 
the tester should familiarize themselves
with the product and the code. It may 
also be helpful to perform research to 
find similar products on the market.