If that field is in the cart, then it is most likely being added as a cart attribute, which becomes an order attribute after purchase.
It looks like on that customer page you have access to the liquid order object, so you should be able to access the order.properties:
https://shopify.dev/api/liquid/objects/order
I tried creating an account but it still wouldn’t let me add stuff to cart, so I can’t get the exact name of the variable, but it would be something like {{ order.properties.variable_name }} to output that.