I do that server side - you can see my instructions for s3 + nodeJS here.
The salient line in the server code section is (note the “Account_Uploads/” - that’s the folder name - if the folder doesn’t exist in your s3 bucket, it will be created - I’m prepending a date stamp to the filename):
Key:`Account_Uploads/${Date.now().toString()}-${JSON.parse(Object.keys(req.body)[0]).filename}`, // what we'll call our file - here I'm using a folder called "Account_Uploads" to put all my uploads into, then prepending a date string to the filename to avoid collisions - S3 will overwrite a file if another with the same key (i.e. name) is uploaded! We have to again extract the filename in a tedious fashion...
Hope that’s helpful.
PS: I should add that I don’t use Companion, so YMMV…