much the same as bridge= and cutting=, split the roadway at the ends of the road and tag that section of road as "embankment=yes"
LINZ:embankment_use could provide more detail, see the Harbour and marine feature wiki pages for tagging coastline as seawalls etc.
embkmt_use values

Value Frequency
stopbank 2420
causeway 89
groyne 5
seawall 3
embankment 1