How get a variable for Node.js in Flutter?

How can I get the ” Jafar ” as a variable in Flutter and set on instead of the ” Jafar” in Node.js?

My code:

const upload = multer({
    storage :multerS3({
        s3:S3,
        bucket:`${config.AWS_BUCKET_NAME}/${'Jafar'}`,
        // bucket:config.AWS_BUCKET_NAME,
        acl:'public-read',
        key :function(req,file,cb){
            if(file.fieldname == "singleFile"){
                cb(null,"single/"+ file.originalname);
            }

            if(file.fieldname == "multipliesFiles"){
                cb(null,"multipliesFiles/"+ file.originalname);
                }

            if(file.fieldname != "multipliesFiles" &&file.fieldname != "singleFile" ){
                cb(null,"veryPersonal/"+ file.originalname);
                }
                // cb(null,file.originalname);
            }
    })
})

Also, I send image.

I tried fined the solution in the internet (YouTube, Github, …) but I didn’t know what to look for in this situation,