Assign host ip from next available IP in reserved range

[#1]

I can currently identify the range by using the <'range?network=192.168.120.0/24'> endpoint. 

I can then find the reserved range based on 'start_addr'. 

let's say range_data['_ref] = 'range/XX5zXmRoX3BfcmXuZ2UkMTkyLjE2OC4xMjAuMjE0LzE5Mi4xNjguMTIwLjIxNy8vLzAv:192.168.120.214/192.168.120.217/default'

I can then get the next available IP using <range_data['_ref'] + '?_function=next_available_ip'>
I want to skip the last step above and both get and reserve the IP in one call (similar to how I currently get an IP from a subnet with no reserved ranges)
endpoint - <"/record:host?_return_fields%2B=name,ipv4addrs">

json_body = {

'name': request_hostname.lower(),

'extattrs': {

'REQUEST': {'value': change_number}, 'TYPE': {'value': 'Server'}},

'ipv4addrs': [{

'ipv4addr': {

'_object_function': 'next_available_ip',

'_result_field': 'ips',

'_object': 'network',

'_object_parameters': {

'network': request_subnet

}

}

}]

}
I would think I would just add something like 'range': range_data['_ref'] under '_object_parameters', but this returns AdmConProtoError: Unknown object type