How to get the Identifier and Secret?

Step 1:

Login to your WHMCS admin.

Step 2:

Navigate to General > API Credentials

Step 3:

Click on Create API Role.

Step 4:

Check all domain and products from Allowed API Actions.

Step 5: 

Navigate to API Credentials and click on Generate New API Credentials. 

Step 6: 

Copy the Identifier and Secret that has now generated.