How to correctly pass the "extra_attributes" parameter to the showPayment function?

Question

Classmates

Hello.
We are currently trying to set up and test payments, but are stuck passing the "attributes" (extra_attributes) parameter when the payment window pops up. In our callback link, something like "Object+Object" always appears. We tried to pass various data formats with json to the showPayment function, but in response we received only object + object. Example from your documentation:
[
{"user" : "2345"}
{"tx" : "ksajdkadsj"}
]
there is a syntax error here, but even after fixing it, it still doesn't work.
How can we pass the "extra_attributes" parameter to the showPayment function?