$ sbx secret set-custom --host api.example.test --env M101_NAMED --placeholder 'sk-{rand}' --value m101-dummy-1111
Applied secret updates for <n> running sandbox(es)
Saved custom secret placeholder "sk-<rand>" for target "api.example.test" env "M101_NAMED" in scope "(global)"
You may need to update environment variable M101_NAMED inside existing sandboxes to "sk-<rand>" (the placeholder value).
[exit 0]

$ sbx secret ls --json
{
  "secrets": [],
  "custom_secrets": [
    {
      "scope": "global",
      "targets": [
        "api.example.test"
      ],
      "env": "M101_NAMED",
      "placeholder": "sk-<rand>",
      "secret": "m101-d*********"
    },
    {
      "scope": "global",
      "targets": [
        "host.docker.internal",
        "localhost"
      ],
      "env": "M101_RECV_KEY",
      "placeholder": "sbx-cs-<rand>",
      "secret": "m101-d**************0000"
    }
  ],
  "shadowed_services": [],
  "env_only_count": 0
}
[exit 0]