The argument type 'Set<String>' can't be assigned to the parameter type 'Map<String, String>' - flutter

Question

I'm trying to create an user authentication system using Auth0 in my Flutter app. The Auth0 REST documentation gave an example of cURL but I didn't find any flutter package which does the job of cURL. So, I used http.
Here's the code:
Future<String> getToken(String userId) async {
final response = await http.post(
Uri.parse('https://my-auth0-subdomain.auth0.com/oauth/token'), // I used my real subdomain
body: jsonEncode({
'grant_type=client_credentials',
'client_id=my_project_client_id', // I used my real client id
'client_secret=my_project_client_secret', // I used my real client secret
'audience=https://my-auth0-subdomain.auth0.com/api/v2/' // I used my real subdomain
}),
headers: {
'content-type: application/x-www-form-urlencoded'
},
);
final token = jsonDecode(response.body)["access_token"];
return token;
}
This gives me an error that The argument type 'Set<String>' can't be assigned to the parameter type 'Map<String, String>'. on line 10 (headers: {...}). I can resolve this error by using headers: {'content-type': 'application/x-www-form-urlencoded'},.
But this then gives the error from Auth0 {"error":"access_denied","error_description":"Unauthorized"}. The API is set up properly, because on running
curl --request POST \
--url 'https://my-auth0-subdomain.auth0.com/oauth/token' \
--header "content-type: application/x-www-form-urlencoded" \
--data grant_type=client_credentials \
--data 'client_id=my_project_client_id' \
--data client_secret=my_project_client_secret \
--data 'audience=https://my-auth0-subdomain.auth0.com/api/v2/'
it returns a "access_token", "scope", "expires_in" and "token_type".
Please help. It's very important.
Thanks in advance :)

Answer 1

Try to send data as url encoded using :
Map<String, String> myBody= {
'grant_type' : 'client_credentials',
'client_id' : 'my_project_client_id', // I used my real client id
'client_secret' : 'my_project_client_secret', // I used my real client secret
'audience ' : 'https://my-auth0-subdomain.auth0.com/api/v2/' // I used my real subdomain
};
Future<String> getToken(String userId) async {
final response = await http.post(
Uri.parse('https://my-auth0-subdomain.auth0.com/oauth/token'), // I used my real subdomain
body: myBody,
headers: {
'content-type: application/x-www-form-urlencoded'
},
);
final token = jsonDecode(response.body)["access_token"];
return token;
}