webwrite returns error 405, how to find its detailed cause?

I am trying to get certain graphiql working, but am getting 405 error without any detailed error description.

url = 'https://rata.digitraffic.fi/api/v2/graphql/graphiql';
query = '{  currentlyRunningTrains {    trainNumber  }}';
options = weboptions('KeyName','Accept-Encoding','KeyValue','gzip');
res = webwrite(url, query, options);

Is it possible to find what is the exact cause for this HTTP 405 error?

P.S. is there any way to properly add

Accept-Encoding: gzip

field into weboptions?