Add variable to postman header

pre-request.js

const this_data = JSON.parse(pm.request.body.raw)
const ref_id = this_data.ref_id
const merchant_id = pm.variables.get('merchant_id');
const secret = pm.variables.get('secret');
const sign_str  = `${secret}-${merchant_id}-${ref_id}`;
const signature = CryptoJS.MD5(sign_str).toString();

pm.request.headers.add({
    key: "HTTP-X-APPAPI-AUTHORIZATION",
    value: signature
});