The first step in your application should be to retrieve and store this string for future use.
a GET request to /administration/users/current will return the following:
{
"AccountId": "00000000-0000-0000-0000-000000000000",
"AccountKey": "",
"Username": "",
"CompanyName": "",
"Email": "",
"Name": ""
}
Full documentation here: https://services.reachmail.net/#resources/Administration/Users/Current
The account key is the friendly account name you use to log into the user interface and can be provided with a request to support@reachmail.com for assistance.