I just took a look in your account and I don't see any signup forms created. Can you please reply with a screenshot of what you are referring to?
Here are the steps
I am navigating Basic Landing Pages.
I have a sign up form created already.
I added a Country as custom field.
This is the preview of my text boxes with country
I saved my changes and copy my sign up form url
When I click my url expected text box is Country but i am seeing it as adress.
I hope it is clear.
Rather than using a custom field for country in your legacy sign up form, I'd recommend selecting the default "Address" option we have available, then removing the other address types until just country remains (see screencap below). That said, since you only have one list associated with your sign up form, it may be better to switch over to the newer Landing Page sign up form builder.