How can I add customer's mobile number to order on Poynt terminal?

You don't have to if the customer chooses to receive receipt by text as shown in How to enable and customize payment receipt on Poynt terminal?

Alternately, to add a customer's mobile number on the Poynt terminal, perform the steps described on the Poynt-Textin integration page.

• Select TEXTIN box at Checkout
• Enter Customer's Mobile# (take verbal permission from the customer or ask customer to enter)
• Customer follows (customer receives a text message on his/her device asking for opt-in)
   

Once step #3 completes, customer is deemed to have completed opt-in to follow your business. Now your Text campaigns would be able to send messages to this customer.