How can I allow open an external acces to a URL

I have a wordpress plugin that needs to be able to send information to another site for payment. I tried to work with the payment gateway developer. They told me to ask Cloudflare this:

Please try to consult Cloudflare , ask them how can you open an external acces to this URL:


I already whitelisted their IP address. I’m not sure what I’m supposed to do next to get this working.