The ID of the Asset you wish to set the custom value for.
The name of the custom field to be set.
The value to be set.
FM_api('Asset_setCustom', ['id' => '4', 'name' => 'colour', 'val' => 'red'])
{"ok":1,"asset":{"type_id":"1","meta":"{\"custom\":{\"colour\":\"blue\",\"\":\"blue\"},\"main\":{\"vehicle_reg\":\"NU11TYP\"}}","location_type":"1","location_id":"3","owner_id":"0"},"asset_type":{"id":"1","name":"Fire","meta":"{\"template\":[{\"name\":\"Main\",\"draggable\":[{\"label\":\"Colour\",\"type\":\"none\",\"left\":20,\"top\":30,\"width\":200,\"height\":50,\"data_type\":\"Single-line Input\",\"field_name\":\"colour\"}]}]}","last_edited":"2018-09-27 12:31:59","attrs":"1","rental_type_id":"2"},"field""...