I seem to have figured it out myself.
In the web service it was as simple as an ordinary $_POST['username'] to get the value !
On the client i changed the request to this:
-(User *) getUserDetailsByUsername:(NSString *)username{
User *newUser = [[User alloc] init];
if([username length] > 0){
NSURL *aUrl = [NSURL URLWithString:@"http://example.com/userbyusername"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:aUrl
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:60.0];
[request setHTTPMethod:@"POST"];
NSString *postString = [NSString stringWithFormat:@"username=%@", username];
[request setHTTPBody:[postString dataUsingEncoding:NSUTF8StringEncoding]];
NSURLResponse *response = nil;
NSError *error = nil;
NSData *data = [NSURLConnection sendSynchronousRequest:request
returningResponse:&response
error:&error];
if(data.length > 0 && error == nil){
NSDictionary *userdata = [NSJSONSerialization JSONObjectWithData:data options:0 error:NULL];
[newUser setUserid:[[userdata objectForKey:@"id"] intValue]];
[newUser setUsername:[userdata objectForKey:@"username"]];
[newUser setAge: [[userdata objectForKey:@"age"] intValue]];
[newUser setFirstname:[userdata objectForKey:@"firstname"]];
[newUser setLastname:[userdata objectForKey:@"lastname"]];
[newUser setEmail:[userdata objectForKey:@"email"]];
}
}
return newUser;
}