This is API from goverment site. And there is nothing about autorization just, as mentioned i have API key from them and i should use JWT Token in header:
"-H “Authorization: Bearer {JWT-TOKEN}”
That is all, is that documentation missing something to create JWT Token?
Okay, well without knowing the specific site, or being able to see their documentation, I suggest reaching out to the support team that manages the resource.
There are a lot of variables to generating a JSON Web Token and what is required is dependent on the resource you are attempting to authenticate to as well as the client making the request and the technology being used.
I would make the assumption that you would have a place to upload a public key for which the resource would use to validate the JWT that is sent to them is signed with the appropriate private key. There are a number of frameworks out there that help you manage/generate such tokens as well as resources on how to do so manually if needed.
But ultimately you need to understand the requirements of the resource you are attempting to use.