A few lines on your server.

import { createHash } from 'node:crypto';
import { CoRelayerClient, type RelayRequest } from '@corelayer/sdk';

// On your server. The sponsor key never reaches a browser.
const apiKey = process.env.CORELAYER_API_KEY;
if (!apiKey) throw new Error('Set CORELAYER_API_KEY');
const corelayer = new CoRelayerClient({
  baseUrl: 'https://api.co-relayer.com',
  apiKey,
});
// Your user signed the transaction. Your plan pays the network fee.
export async function payForUser(req: RelayRequest, actionId: string) {
  const key = createHash('sha256').update(actionId).digest('hex');
  const { data } = await corelayer.relay(req, { intentKey: key });
  return data.txHash;
}