How can I include a placeholder in a localization message in Flutter? 

You can use the ICU Message syntax to include placeholders. That will allow you to handle almost anything, from a simple text input to formatting numbers, dates, times, plurals, and other data types.

Localization message in arb file:

{
  "stringKeyExample": "Welcome {name}",
  "@stringKeyExample": {
    "placeholders": {
      "name": {}
    }
  }
}

Access the localization message in code:

AppLocalizations.of(context)!.stringKeyExample('John')