For XML/JSON, follow steps below:
You can authenticate like this:
http://yourservername:portnumber/psa/api.do?function=login&username=[username]&password=[password]&company-code=[company code]
If data format is XML and your authentication is successful, you will receive XML with two elements: token and user_id. For example:
<result> <token>a1dc42fb-d420-10feb-aaks9-89f5c0e8sk3c</token> <user_id>112233</user_id> </result>
In case the data format is JSON, you will receive an object with properties token and user_id respectively.
{"result":{"token":"a1dc42fb-d420-10feb-aaks9-89f5c0e8sk3c","user_id":112233}}
On how to handle errors, see Error Handling and Debugging