Simple utility to schedule webhooks with at specific time and url

I am building a serverless application where in every invocation of a serverless function, I would like to schedule another future call with different payload and url. I am using Vercel to deploy the lambdas.

However, I am not able to find a simple way to just schedule the next function call, ie, an online utility that just takes (datetime, url, payload, headers) as inputs and makes an API call at the specific date time.

I had checked Cronhooks but it seemed quite expensive (5cents per webhook!). The value of these webhooks is not that high for me, hence looking for something much cheaper