To add a promotion to a customer in POS this can be done by:
After selecting the table with the desired products, you need to click on the Option button (the 3 dotted circle). 
After that, select the Loyalty/Promotions option.
Attention: Depending on how the promotion was created, you can choose to search the client that you know it has a promotion assigned to it.
Or, if you have created a promotion that is applied manually without loyalty but you still want to assign a client to it, you can search for the client, and then click on Promotions to select your desired promotion.
After you have selected the promotion, the amount will be reduced automatically and you will only have to print the fiscal receipt.
To find out more how can you set up promotions, please Click here