Hi Jack,
Typically API_DoQuery and an Webhook are not compatible for the use you are looking to put them to of getting data from somewhere else and then including it in a change. This is because Webhooks are only a one way communication, they send out a request but they aren't set up to take the response back and do anything with it. That is usually an example of where a developer needs an intermediate in the service to send out the API_DoQuery, get back the request, and then send the now finished and formatted data into Quick Base. Webhooks don't have the capacity to handle multiple steps like that.
For your Contact table do you have the key field set to email address? If you did then you could set up the relationship between Contacts and Opportunities to use the related contacts email to identify them automatically. Then you would just push the email address that comes in with the Webhook to write into the related contact field so long as they key on Contacts is email address. That just requires you to change the key field if you don't currently have it set to email address which can break any other relationships you have and would make them need to be rebuilt. I hope that information is helpful Jack and thank you for reaching out.
