See X-Request-ID in error messages of PostgreSQL
Postgres knows nothing about how your application generates the SQL requests that are sent to the database engine. So it also has no idea that there is a HTTP request behind it, let alone the header of such a request. 

The information that can be put into a log line is fixed and documented in the manual 

The only "log variable" that you can dynamically change, is the application name. However that requires that your application sets that each time it processes a request. 

Before running a SQL query, your application needs to run something like this: 

set application_name = 'request-42';


where 'request-42' is the request ID you extracted from the HTTP request.

Once you have that, you can add the %a to log_line_prefix and it will be shown in the log file.