As Philip stated there is not an endpoint to get the same information that is available for the MS. The closest you can get on the MX is the /networks/{networkId}/appliance/ports endpoint, but this doesn’t provide interface bandwidth/duplex configuration.
(Since you have to use the local status page to get the bandwidth/duplex on a MX and it can’t be changed through the Dashboard, I wonder if this is a shortcoming of the Current MX firmware?)
EDIT: Be aware that the capabilities of a MX switch port are far less than those of a MS switch port.