The attr_write requires the attribute type (an unsigned 8 bit number).
In your case it an uint8 - which is cod 0x20 or 32 in decimal.
service: zha_toolkit.execute data: command: attr_write # You could replace this with the entity name if you like: ieee: cc:86:ec:ff:fe:d8:43:b6 # Endpoint can be omitted if there is only 1 cluster of the given type on the device endpoint: 1 # A bit suprised that cluster 0 is used for enabling/disabling a siren, but that's the doc. cluster: 0 # Ok, you could also write this directly as 0x400a attribute: 16394 # Ok, you cuould also omit the quotes attr_val: '1' # The type is required when writing attr_type: 0x20 # The attribute is manufacturer specific - fill in th emanufacturer id. manf: 0x???? # Optioinal, setting an event to get an idea of what happened from the UI. # You can "Listen" to the 'my_write_event' from Developer Tools > Events event_done: my_write_event