curl --request PATCH \
  --url https://api.merchantops.ai/api/pricing/proposals/{key}/link-variants \
  --header 'Content-Type: application/json' \
  --data '
{
  "variant_keys": [
    "<string>"
  ]
}
'

{
  "detail": [
    {
      "loc": [
        "<string>"
      ],
      "msg": "<string>",
      "type": "<string>",
      "ctx": {},
      "input": "<unknown>"
    }
  ]
}