Read proper write-up here: https://publish.whoisbinit.me/subdomain-takeover-on-api-techprep-fb-com-through-aws-elastic-beanstalk
I have included my script in another file (main.sh), which I used in discovering this vulnerability.
I didn't do any form of manual work in finding this vulnerability, and my workflow was fully automated with Bash scripting.
I have shortened my actual script, and only included the part which helped me in finding this vulnerability in the main.sh file.
Thank you for your quick answer. There's something odd, that dialog appeared when I created the second environment and not while creating the first.