How do I set the directory or file path in S3 using AWSS3GetPreSignedURLRequest?

Looking at the AWSS3GetPreSignedURLRequest class, I don't see any way to specify a directory in my bucket.

For example, there is a directory images inside my bucket and a file named "first.png". I can set the key to "first.png" but that would only work if the file is on root directory of my bucket.

When I try to set the key to "images/first.png", it does not work either. How do I target files inside a directory using AWSS3GetPreSignedURLRequest?

Thanks