The best way to troubleshoot this is to use global searches and check the selfLink fields to figure out the right syntax
For example, run some of these commands and check the output.
curl -sk -u admin:admin https://F5HostName.com/mgmt/tm/ltm/pool/ | json-format
curl -sk -u admin:admin https://F5HostName.com/mgmt/tm/ltm/pool/~<partition>~<poolname>/members/ | json-format
You would see the right syntax of each pool member (with the correct partition) in the selfLink.
"selfLink": "https://localhost/mgmt/tm/ltm/pool/testpool/members/~Common~192.168.101.11:8000?ver=11.5.1"
REF - https://devcentral.f5.com/s/articles/icontrol-rest-working-with-pool-members
KR,
Dario.