Possible to access a fetch/request body in the browser and send it too?

If there is another community this belongs in instead, please point me in the right direction. Thank you.

I am trying to hash and sign my fetches, unfortunately, it does not seem possible to do that. I finally figured out that I can create a request and access the data I need that way. However, when I attempt to access the body in any way I get the error that the body has already been consumed.

Is there any way around or through this problem where I can access the final result of the body while still being able to send the request afterword.

What I am actually trying to accomplish is to hash the body and sign it with a local keypair, then append the signature as a header. Then send the request to be validated and acted upon serverside. Everything except the hashing and accessing the final version of the body I have working. Unfortunately, I need to access the final form because with at least multipart forms, there is additional data added that I need access to for a correct hash.