YOu need to use the 'interface' endpoint with 'link-subnet': e.g.
maas admin interface link-subnet d77f3f 63 mode=AUTO subnet=10.248.64.0/19
YOu need to use the 'interface' endpoint with 'link-subnet': e.g.
maas admin interface link-subnet d77f3f 63 mode=AUTO subnet= 10.248. 64.0/19