what you are looking for is encryption and decryption. which already happens for sites hosted on https: only the frontend and the server knows the content of the request and the response

I want to hide the request body

For?

Depending on the use case, it could be possible or not possible. What are you trying to do?

I don't really think you can that well , any encryption function can be found in the JS files

But I'd like to know why you want to do it

can not I use the brotli compression to compress my request body ??

“No one can see this” then brotli is not the solution. Everyone can uncompress brotli