Plesk Expand allows you to provide a single access point for all clients, domain administrators and mailbox owners. After logging in, these customers will be automatically redirected to their accounts - Plesk Expand defines what Plesk server a certain customer is located on and redirects him/her to their account on that server.
Thus, you do not have to tell your customers what Plesk server exactly their accounts reside on. You can either provide your customers with the URL to your Plesk Expand, so they could use the Plesk Expand login form to log in to their accounts, or you can build your own login form, place it on one of your web sites and provide your customers with the URL to this web site.
In the login form, your customers should enter the data specific for each user type:
The single login form should be build on the basis of a GET submit type form. The following two points should be taken into account:
Here is the example of the single login form:

The schema for building the provided GET submit type form is as follows:
<html>
<body>
<form name="main"
action="https://expand.swsoft.com:8442/plesk/unilogin.php" method="GET">
Login: <input type="input" name="login" value="" size="40" maxlength="200">
<br>
Password: <input type="password" name="password" value="" size="40"
maxlength="200">
<br>
<input type="submit" value="Login">
</form>
</body>
</html>