Currently logstash supports the api key in the form of "id:key":

input {
  elasticsearch {
    "api_key" => "TiNAGG4BaaMdaH1tRfuU:KnR6yE41RrSowb0kQ0HWoA" 
  }
}

However, the Elasticsearch output when creating an API Key is:

{
  "id" : "9tPacX4BGhtIQNIMawN9",
  "name" : "my-api-key",
  "api_key" : "hATekzVwRemmgCSxJXcWsw",
  "encoded" : "OXRQYWNYNEJHaHRJUU5JTWF3Tjk6aEFUZWt6VndSZW1tZ0NTeEpYY1dzdw=="
}

Which means a user must compose the "id:key" themselves. Using the encoded form would mean less copy pasting, less errors, and simpler experience.
For backwards compatibility sake we can still support the "id:key" format by detecting if the input has a ":". In this case we can skip the base64 encoding and use the input as is.