Modifying Pool Members through REST API

can you try something like this?

 rest call

[root@ve11d:Active:Changes Pending] config  curl -sk -u admin:admin https://localhost/mgmt/tm/ltm/pool/foo/members | /var/tmp/jq .
{
  "kind": "tm:ltm:pool:members:memberscollectionstate",
  "selfLink": "https://localhost/mgmt/tm/ltm/pool/foo/members?ver=11.6.0",
  "items": [
    {
      "kind": "tm:ltm:pool:members:membersstate",
      "name": "200.200.200.101:80",
      "partition": "Common",
      "fullPath": "/Common/200.200.200.101:80",
      "generation": 1,
      "selfLink": "https://localhost/mgmt/tm/ltm/pool/foo/members/~Common~200.200.200.101:80?ver=11.6.0",
      "address": "200.200.200.101",
      "connectionLimit": 0,
      "dynamicRatio": 1,
      "ephemeral": "false",
      "fqdn": {
        "autopopulate": "disabled"
      },
      "inheritProfile": "enabled",
      "logging": "disabled",
      "monitor": "default",
      "priorityGroup": 0,
      "rateLimit": "disabled",
      "ratio": 1,
      "session": "user-enabled",
      "state": "unchecked"
    }
  ]
}
[root@ve11d:Active:Changes Pending] config  curl -sk -u admin:admin https://localhost/mgmt/tm/ltm/pool/foo/members -X POST -H "Content-Type: application/json" -d '{"name":"200.200.200.111:80"}' | /var/tmp/jq .
{
  "kind": "tm:ltm:pool:members:membersstate",
  "name": "200.200.200.111:80",
  "fullPath": "200.200.200.111:80",
  "generation": 9,
  "selfLink": "https://localhost/mgmt/tm/ltm/pool/foo/members/200.200.200.111:80?ver=11.6.0"
}
[root@ve11d:Active:Changes Pending] config  curl -sk -u admin:admin https://localhost/mgmt/tm/ltm/pool/foo/members | /var/tmp/jq .
{
  "kind": "tm:ltm:pool:members:memberscollectionstate",
  "selfLink": "https://localhost/mgmt/tm/ltm/pool/foo/members?ver=11.6.0",
  "items": [
    {
      "kind": "tm:ltm:pool:members:membersstate",
      "name": "200.200.200.101:80",
      "partition": "Common",
      "fullPath": "/Common/200.200.200.101:80",
      "generation": 1,
      "selfLink": "https://localhost/mgmt/tm/ltm/pool/foo/members/~Common~200.200.200.101:80?ver=11.6.0",
      "address": "200.200.200.101",
      "connectionLimit": 0,
      "dynamicRatio": 1,
      "ephemeral": "false",
      "fqdn": {
        "autopopulate": "disabled"
      },
      "inheritProfile": "enabled",
      "logging": "disabled",
      "monitor": "default",
      "priorityGroup": 0,
      "rateLimit": "disabled",
      "ratio": 1,
      "session": "user-enabled",
      "state": "unchecked"
    },
    {
      "kind": "tm:ltm:pool:members:membersstate",
      "name": "200.200.200.111:80",
      "partition": "Common",
      "fullPath": "/Common/200.200.200.111:80",
      "generation": 9,
      "selfLink": "https://localhost/mgmt/tm/ltm/pool/foo/members/~Common~200.200.200.111:80?ver=11.6.0",
      "address": "200.200.200.111",
      "connectionLimit": 0,
      "dynamicRatio": 1,
      "ephemeral": "false",
      "fqdn": {
        "autopopulate": "disabled"
      },
      "inheritProfile": "enabled",
      "logging": "disabled",
      "monitor": "default",
      "priorityGroup": 0,
      "rateLimit": "disabled",
      "ratio": 1,
      "session": "user-enabled",
      "state": "unchecked"
    }
  ]
}

 tmsh

[root@ve11d:Active:Changes Pending] config  tmsh list ltm pool foo
ltm pool foo {
    members {
        200.200.200.101:80 {
            address 200.200.200.101
        }
        200.200.200.111:80 {
            address 200.200.200.111
        }
    }
}