How do i assign my custom 404 page to be the real http 404 status ?

I've created a custom 404 page, I've assigned it under the 404 system tools in Squarespace. However when i check the status of the http for that URL, it shows as 200 instead of 404. 

anyone else have this issue and know how to resolve it ? 
Screenshots attached, thankyou

This is the page i created and would like to use: 
https://www.redcatdigital.au/404-page