The record must be referenced using one of its identifiers (code, internal ID).
Therefore, to add an external ID to an existing record, a JSON array must be used.
Example:
{
"winstrom":{
"@version":"1.0",
"faktura-vydana":[
{
"id":[
"ext:123",
"code:VF1-0035/21"
],
"popis":"popis"
}
]
}
}