This tutorial assumes you've already logged in to WHMCS
Now let's learn how to register a domain name
1) Click the Portal Home link

2) Click Order
 
3) Click the Register Domain link

4) Enter a domain name here

5) Choose the top-level domain

6) Click Check Availability
 
7) Click Add to Cart
 
8) Then click Update Cart
 
9) Click Checkout
 
10) Choose your payment method, then click Complete Order
 
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This is the invoice for the domain we just registered

11) Click Back to Client Area
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That's it! Â We have registered a domain name
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12) Click the My Invoices link

13) We can see the invoice we just created here which is currently Unpaid
 
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This is the end of the tutorial. Â You now know how to register a domain name in WHMCS