How make a http post using form data in flutter?

Use Map instead, because body in http package only has 3 types: String, List<int> or Map<String, String>.

Try this:

final Uri uri = Uri.parse('https://na57.salesforce.com/services/oauth2/token');
    final map = <String, dynamic>{};
    map['grant_type'] = 'password';
    map['client_id'] = '3MVG9dZJodJWITSviqdj3EnW.LrZ81MbuGBqgIxxxdD6u7Mru2NOEs8bHFoFyNw_nVKPhlF2EzDbNYI0rphQL';
    map['client_secret'] = '42E131F37E4E05313646E1ED1D3788D76192EBECA7486D15BDDB8408B9726B42';
    map['username'] = '[email protected]';
    map['password'] = 'ABC1234563Af88jesKxPLVirJRW8wXvj3D';

http.Response response = await http.post(
    uri,
    body: map,
);