Wordpress login from windows application

Hi,
We have a wordpress site and the paid members can access all our online HTML5 games and/or use our windows applications without any time restrictions. In order to do so, they have to login to the website. So far we have been using the Web Control Object for our windows applications to check the level of the members.
I have recently tried to improve the login system using the GET object with the hope that it would be faster and without the warning messages that appear while trying to login.
But it has not been possible to POST the user name and the password with the default system of wordpress.

The first method I tried:
Add Post date "user_login" (this is the ID of the user name text box)
Add Post date "user_pass" (this is the ID of the password text box)
Get URL Please login to see this link.
On get complete I receive a login error mentioning that the text fields are empty.

The second method I tried:
Set user to "member user name"
Set password to "member password"
Get URL Please login to see this link.
On get complete I receive the html code as if the text fields were not filled.

I made an external authentication system using the instructions on this page:
Please login to see this link.

This time it worked fine and amazingly fast but the problem is that the login is not recorded by the wordpress system so it is not possible to control simultaneous logins. If we fail to control this, then we cannot prevent sharing login credentials by users.

Any ideas on how to use the default system of wordpress using the GET object?
Maybe this is possible using the Web Query Object instead?
Any help would be much appreciated!

Thank you Danny,
I am already using a membership plugin. The problem is that when I use the external authentication system, the login is not recorded. I can GET the membership level and stop the time restrictions but there is no record of the login. So any number of users would be able to use the same user name and password at the same time.
The only way to record the user's login is the standard login system of wordpress. So far I have not found a way to use the standard wordpress user_login and user_pass fields.
I tried different approaches with php and simple html like this Please login to see this link. but the login is recorded only when I use the browser. When I use the GET object from my application nothing happens!
The best solution I can think is using the external authentication system first and if the user is a member then make a second GET action to another php/html so that the login will be recorded. I am still trying to find out how ...

Thanks again Danny!
I am in a dead-end so far but I will keep trying!

I am giving up!
Although using the GET object is much faster (not more than 4 seconds) and it gives me the level of each user very easily, I could not find away to "count" the number of simultaneous logins for each user. The reason is that the custom login system is not detected by the default wordpress system and no workaround I tried made it work!
I have decided to use the Web View Object instead of the Web Control Object I have been using so far, because it returned script error messages to the user and made the customer experience poor.
The problem with the waiting time still remains (between 19 and 26 seconds just to show the page).