To create a new custom property for an item or update an existed one, use the command item/update_custom_property:

svc=item/update_custom_property&params={"itemId":<long>,
					"name":<text>,
				 	"value":<text>}

To delete custom property, use the same command but with empty value param:

svc=item/update_custom_property&params={"itemId":<long>,
					"name":<text>,
				 	"value":""}

{
	"n":<text>,	/* custom property name */
	"v":<text>	/* custom property value */
}