How do I add a payment method in WooCommerce programmatically?

You can get a developer to do it or you can use this post here - https://stackoverflow.com/questions/60369563/create-a-woocommerce-order-programmatically-and-apply-payment-method - as an automatic script for it.

This hyperlink also has good information on WooCommerce payment gateway plugins for you

Some people do it for free as a courtesy to others - but as with ALL THINGS FOUND ONLINE - be careful and check your sources. Something like this is raw code and can be directly checked to scan for malicious code, be careful for any plugins or extras when using these sources.