For authentication, you must provide your API username and password. This is different to the account username and password that you use to log on to the web interface.
You can find your API username and password via the web interface, Message Box:
http://messagebox.textmarketer.co.uk/ (Account Settings > API Config tab)
There are a couple of ways to pass this information to the API. The first is via the GET/POSTvariables. For example you could put them in the GET variables like this:
http://api.textmarketer.co.uk/services/rest/credits?username=u&password=p
When using the POST action, you would need to add the username/password variables to the POST arguments instead.
The second option, for those who prefer it, is to use HTTP Basic Authentication.
Security
All the REST API services are available using an SSL connection, i.e. using https URLs instead of http.