I took a peek at the Bittrex plugin code, and I saw this:
Code
DLLFUNC int BrokerLogin(char* User,char* Pwd,char* Type,char* Account)
{
	if(User) { // login
		memset(&G,0,sizeof(G));
		G.Amount = 0.00001;
		strcpy_s(G.Key,User);
		strcpy_s(G.Secret,Pwd);
		if(!*User || !*Pwd)
			showMsg("Prices only","");
		else if(!BrokerAccount(Account,NULL,NULL,NULL))
			return 0; // invalid credentials
		else 
			showMsg("Bittrex plugin",PLUGIN_VERSION); // all ok
		return 1;
	} else { // logout
		if(G.HttpId) 
			http_free(G.HttpId);
	}
	return 0;
}
As you can see, it is calling BrokerAccount, using Account as an input.

The documentation says that char* Account is an optional output of char[1024]. Is it also an input now? Does it pull from the Accounts.csv file then?