Step::1 Login to WHMCS admin area.
Step::2 Navigate to Setup section and then click on Manage API credentials under Staff Management section.
Step::3 Before generating API credentials you need to set API rules first. So Click on API roles tab.
Step::4 Now click on Create API role.
Step::5 In the next step a dialog screen will pop-up in which you have to fill the details and proceed.
Step::6 After creating API rules now go to API credentials tab and click on the Generate New API Credential button to open Generate New API Credential dialog screen.
Step::7 You must select the admin member of staff for whom the new credential will authenticate.
Step::8 Click the Generate button to provision a unique API credential for the selected admin user. The dialog screen will update and provide the credential Identifier and Secret.
*You must copy the Secret value. It will not be available at a later time due to cryptographic security. If you fail to copy this value, a new credential pair will need to be generated.
That's It !
Check out the video tutorial on the same to get detailed explanation. Read the blog as well.