How to make payment gateway app in xamarin forms

A couple of things before you start ...

1) Never, ever, accept code from a insecure website to handle anything to do with real money.
You do not know who is giving you the code, you do not know what it does, you do not know that it places the monies correctly into the appropriate account, without passing the details to any third parties.

Only get such code from reputable card transaction service companies - the scope for fraud otherwise is far too large. And remember, you personally could be liable for any monies lost if your action is seen to be negligent - which getting your code from a public forum would most certainly be!

2) Security is a major part of anything dealing with money, as is Data Protection: unless you have a solid grounding in both fields, you will almost certainly leave yourself open to major problems - just like getting code from a public forum. It's far too easy to make assumptions about security that are wildly far from reality - for example the "obviously secure" thing to do with passwords is to encrypt them, but in the real world that is as secure as a chocolate teapot because the decryption method and key must be available in your code where it can be looked at by anyone interested.

I would strongly advise that if you don't know what you are doing - and your extensive question history says you don't - that you find a different project, that probably won't land you in prison or with massive fines ...