How to set a field column of type person for a folder in Sharepoint via the REST API

On Sharepoint I create a column of type person/group (lets say "projectlead"). Now I want to create folders and after the creation I want to set this column via the graph api.

I already tested this with a column of type text (single line) successfully via:

POST https://{{tenant_url}}/sites/{{site}}/_api/web/lists/getbytitle('Dokumente')/items({{ItemId}})/ValidateUpdateListItem()

header:

• Accept: application/json;odata=verbose
• Content-Type: application/json;odata=verbose

body:

{
  "formValues": [
    {
      "FieldName": "projectlead",
      "FieldValue": "someUserIdOrName"
    }
  ],
  "bNewDocumentUpdate": false
}

I heard that it is not possible for folders, but couldn't finde anything specific about it.

Is it possible and if, how? I tried several things, but nothing worked for me.