I think DHW boost is ID 115: HDWStartOneLoading
curl -u "admin:admin" "http://<ECONET_IP>/econet/newParam?newParamName=115&newParamValue=1"
Here’s the full paramater list: econet-param.md · GitHub
I’ve not tested it since I don’t use econet any more. I use Econext which makes integration with HA much easier: Econext: Direct local control of Grant HP290 Econet replacement (Plum ecoMAX360i)
Another option is this Econet HACS integration, this may support DHW boost: GitHub - jontofront/ecoNET-300-Home-Assistant-Integration: ecoNET 300 integration to HA · GitHub