I don't need your money, i just wanna help.
There's no methods to know the subdomains of specific domain, insteed of tricks. And there's 1 trick, it's brute force attack, it has a relation with computing security. And the website that you put in the description of this project does the same method or trick. The brute force is based on dictionary file that contains a list of the possible words of subdomains. Actually there's a program called dnsmap, it contains 2 files, the wordlist file that contains subdomains and the script which you run to test all the wordlist with the domain name given.
And if you don't like dnsmap, i can program for you an easy script that can do the same thing as dnsmap. That's all i can do for you, and at the end, RAMADAN KARIM.