The way of changing the FS:RAM quota fromECE 2.x is to create an instance configuration and then use that - see https://www.elastic.co/guide/en/cloud-enterprise/current/ece-getting-started-templates.html
(To respond to the original question, for completeness -
There isn't a data section in the POST (create) cluster call.
Instead there is a settings object at the top level which contains all the parameters we allow users to change via the v1 API ... note this does not include the FS:RAM override, which should be handled as above, though it remains in our "internal API" which is what the advanced cluster data editor exposes)