flutter login with dio package - flutter

Question

i try to login to the api with dio and i cant handle it
my scenario is:
login with user name,password and server returned session like this:
{
"function": "request_session_generate",
"params": {
"username": "myuser",
"password": "mypass"
}
}
and when send request post method returned session like below code:
{
"result": 0,
"data": {
"session": "mysession...",
"session_status": "generated"
}
}
and second post method for validate this session ... put the session and otp code(google authenticator) like this:
{
"function": "request_session_validate",
"params": {
"session": "mysession",
"otp": "123456"
}
}
when put the session in to the session params server returned:
{
"result": 0,
"data": {
"session": "newSession",
"session_status": "validated"
}
}
how can get the session in the body response first method and use it in the second function for validate
my first function implement code :
class ApiClient {
final Dio _dio = Dio();
Future<Response?> login() async {
try {
Response response = await _dio.post(
'myserver/api',
options: Options(
headers: {
"apikey": "00000000-0000-0000-0000-000000000000",
},
),
data: {
"function": "request_session_generate",
"params": {
"username": "myuser",
"password": "mypass"
}
},
);
//returns the successful user data json object
if(response.statusCode == 200){
return response.data;
}
} on DioError catch (e) {
//returns the error object if any
return e.response!.data;
}
return null;
}
and my implemention for validate and not worked:
Future<Response?> validate() async {
try {
Response responsevalidate = await _dio.post(
'myserver/api',
data: {
"function": "request_session_validate",
"params": {
"session": "mysession",
"otp": "123456"
}
},
);
//returns the successful user data json object
return responsevalidate.data;
} on DioError catch (e) {
//returns the error object if any
return e.response!.data;
}
//IMPLEMENT USER LOGIN
}
how can get session in first function and use it in second function for validate?
use the json decode or something else ?
thank you for help me