How to make a call to another server with a secret

I need to add a form to my website which collects a users name and email, then accepts payment via credit card (to square if possible), then when the payment is complete make an api call to another server, passing the name and email.  The call to the other server will include a secret, so it should be made from the backend of squarespace.  I am long time full stack developer and have developed similar things on wix and woocommerce, but not sure how to start with squarespace.  Can anyone direct me at some resources?  Or, suggestions given the limitations of squarespace?

I don't believe you can do this in squarespace without maybe a 7.0 website that is in developer mode. I still am not entirely sure this is possible though since we don't quite have the same back end server access that another type of site does. Keep in mind though that dev mode adds a whole slew of new potential issues like not getting template updates anymore, extremely minimal SS support on custom code, the issue of handing it off to another dev who doesn't know how or doesn't want to work in dev mode. It's worth checking in with the SS support team maybe to ask or maybe someone else will chime in...

1 hour ago, seth4520 said:

I need to..,make an api call to another server, passing the name and email.  The call to the other server will include a secret, so it should be made from the backend of squarespace.

This is not possible on Squarespace. You only have frontend access.