Create an account on Rinkeby Network. Use MetaMask for it.
Get test ETH on Rinkeby Faucet
Follow the link and register
You know the source code, find it a vulnerability. Use a test environment for this, we recommend such tools, such as Remix IDE or Truffle Framework + Ganache.
In order for the solution to be considered, you need to cause a vulnerability in the address you receive after clicking the button in p. 3.
We advise you to do this via Remix IDE using MetaMask (At address - call the function and pass the received address there)