Install Samsung Pay widget onto the watch. Setup whatever payment methods you want to use on the watch, but do this on the phone. Then those payment options should appear on the watch, however you must first validate those payment methods before they can actively be used on the watch. For me this involved choosing the option "call bank". Your phone will call the appropriate number and you will talk to a human who will ask you questions about recent transactions and their location that occurred using the payment method in question. This is to verify that you are the card holder. Once you have convinced said human that you are not scamming, the payment method gets turned on for use on the watch.