Password for webpage made in squarespace

My website is built on squarespace and I need to restrict access to a webpage. When someone lands on the webpage I want a pop up to block the page and ask the user for a 6 digit code. These codes will be supplied and the system should check the input against the predetermined list of codes. If the inputted code matches one from the list, the user can access the webpage, if not an error message should show up. Functional spec – how it works ----- User clicks a ‘register now button’ A new short webpage or pop up should appear asking them to input a code. (This does not need to be a pop up, we can redirect to another page on Squarespace and build the input field there, but the best UX is desired) The user inputs their code The system checks the inputted code against the provided secure CSV file (location and hosting are part of scope). Codes are alphanumeric, between 4-10 characters long. If the code matches one on the provided list, they are redirected to predetermine checkout webpage If the code does not match one on the provided list, they will see an error message (copy TBD) We need to be able to remove codes and upload new codes. If we could produce a report of codes used, that would be ideal. The code needs to integrate with the Squarespace system and be able to be applied to multiple webpages. Needs to integrate with the Squarespace design template we have installed.