Uploading a verification file on Webflow? (Help!)

Hi, recently i have been trying to create an Alias domain on Rebrandly leading to the main domain (nextlevelacademy.io) which I have been using as my main site. As I progress through the steps, I was instructed by Rebrandly that in order to pass the domain verification I will have to follow these instructions (Picture Below)(https://support.rebrandly.com/hc/en-us/articles/360011811018) to manually upload a verification file.

I’m currently stuck at this step and unable to proceed. Can anyone guide me through this process to upload a verification file on Webflow?