To research a field within Eikon Data API, from your Eikon GUI, run Data Item Browser,
select your instrument, and start typing what you guess to be the main part, the essence, of the content you are looking for, in this case "matur".
You should see zero or more hits, in this case TR.FIMaturity Date. Based on the description and the content displayed, select the relevant, for example:
If you are not able to find what you are looking for, the next best step for content research is Refinitiv Conten Helpdesk, specify Content -> Eikon and provide all the relevant information so that we can best assist you in pinpointing the content.