You can authenticate as follows only using an administrator privileges. This token then needs to be passed in the API query that you will execute. To see how to pass the token, refer here.
http://yourservername:portnumber/psa/api.do?function=login&username=username&password=password&company-code=company code
For example:
http://acmeindia:8888/psa/api.do?function=login&username=janet&password=diamond45&company-code=acme_com
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