espocrm-base/application/Espo/Resources/metadata/fields/foreign.json

{
   "params":[
      {
         "name":"link",
         "type":"varchar"
      },
      {
         "name":"field",
         "type":"varchar"
      }
   ],
   "filter": true,
   "notCreatable": true,
   "fieldDefs":{
      "skip": false
   }
}