How to present this string as a GraphQL query
curl -H "Authorization: bearer token" https://api.github.com/graphql
Help me please
You can follow "Forming Calls with GraphQL", which involves "Creating a personal access token for the command line" to get a token.
Your header should then be: Authorization: bearer xxx, with xxx being the token you have generated.
As a concrete example: "Get started with GitHub GraphQL API".