For API authentication Public and Private keys are required.
Each user has his own unique keys. 

To generate and assign keys to user:

• Login to Admin Area 
• Click User tab
• Click the Options icon under Actions to display the dropdown list, and then click Details
• Click API Settings, it will open for with API Settings
• Check API Access to allow user API requests or uncheck to disalow
• Click Generate API Keys if you assign keys in first time or Reset API Keys if updating 

Important: if you reset API keys it will affect all current scripts for this user