Change Payment Processor in AngularJs

I want to implement external payment in an AngularJs 1.6 webapp.

It has a NodeJs backend with MongoDb, for the internal API;
FrontEnd is written in AngularJs 1.6. (running on Node 10.24.1)

I need to implement Nexi Easy Payment (in italian "Pagamento Semplice")

These are the Nexi docs:
- Examples in php https://ecommerce.nexi.it/specifiche-tecniche/codicebase/avviopagamento.html
- Official technical spec: https://ecommerce.nexi.it/sites/default/files/technicalspecifications.NEXI.18.pdf

Contact me if interested
Best regards