I created a custom field in time entries and this when I get a time entry:

Url: server/api/v3/time_entries/1

{
   "id": 1,
   ....
   "_links": {
      ....
      "customField1": {
          "title": "Test1Option2",
          "href": "/api/v3/custom_options/2"
   }

When I call the link /api/v3/custom_options/2 , I get 
{
    "_type": "Error",
    "errorIdentifier": "urn:openproject-org:api:v3:errors:NotFound",
    "message": "Die angeforderte Ressource konnte nicht gefunden werden."
}