Login using your USER1 credentials.
Default user name is USER1 (all caps) and password (1111).
Click on function Key assignment. The Function Key refers to the button numbers on the phone as you read from left to right and top to bottom.
Under the drop down menu under the function select 01 - DSS/One Touch of the button you wish to change.
The additional data will be the extension number you wish for the button to call.
Note: the line key name is only used on desi less phones. You probably do not have these.