you have to create your own web service at crt level and this service you consume at POS UI.
If you have third party web service, in that case, you have to consume it at crt and send the result using your custom crt service which can be consumed easily at POS UI.
I am not sure about directly can we consume the outside web service in MPOS or cloud pos. But in that case, you have to think in an extreme level of coding in typescript.
Regards Parveen Dhawan.