Step 1:  Click on this link :

Step 2: Click on Continue and it'll open the new tab.

Step 3: Click on Connect WhatsApp Business API Number.

Step 4: Login with your Facebook Account.

Step 5: Click on Continue 

Step 6: Click on Get Started.

Step 7: Click on Continue.

Step 8: Fill the Business Details and click next.

Step 9: Click on next.

Step 10:  Complete the WhatsApp Business Profile Details and Next.

Step 11: Verify your WhatsApp API Number.

Note:  Make sure this number does not have a WhatsApp account.

Step 12: Once Verification is done Click on Continue .

Step 13: Now click on finish and close all the tab.

Final Step:

Go to WhatsTool Business-- Settings---WhatsApp API Number and click on Sync.