Sample signature generation

test.js

const crypto = require('crypto'); // Standard nodejs library. Tested using v10.15.0 and v8.11.4

const sparkcentralSecret = process.env['SECRET']; // shared secret as on settings

const expectedSignature = "1d051f7bfd06c7a3702788bb38bbb967109b5389c1f25af4909d0cb7d75d096d"; // take from request header.

const body = `
{ 
    "email": "fj@example.com"
}
`; // Note: do not parse as json and convert it back to string. Take literally as string from the body including whitespace.

const actualSignature = crypto
  .createHmac("sha256", Buffer.from(sparkcentralSecret, "hex")) 
  .update(body, "utf-8")
  .digest("hex");

if (actualSignature !== expectedSignature) {
  console.error(`Signatures don't match. Expected ${expectedSignature} but got ${actualSignature}`);
} else {
    console.log("Signatures match!")
}

Can be run using node test.js or via docker run -it --rm -v "$PWD":/app node:lts node /app/test.js. You should get following output: Signatures match!. Body should match exactly (do not remove spaces or whitespace).