Clarified URL Signed and Object Loading

I am a little confused how to create a signed url so that a user can upload a file or set of files to gcs. I have successfully created a signed url to access a private object, but now I would like to try the same for multipart upload. It looks like the POST request cannot be signed, so I tried the PUT request, but it didn't work. I'm not sure if I need to use a different endpoint. Also is it possible to create a signed url without specifying the object name first? By the way, I am using postman to check the download process and go to sign the url.