Do you have Application Programming Interface (API) by which we can integrate QuickBook with third-party/legacy applications?

Hello @turquoiseyez-yah,

We have an Intuit Developer forum where you can post questions like yours. One of your developers will answer questions related to programming questions or API's.

You can reach them through this link: https://developer.intuit.com/app/developer/homepage.

I'll also share with you our Intuit Developer blog to keep you updated on the latest program release.

Please don't hesitate to swing by anytime if you have other questions or concerns. We're here to lend a hand. Thanks for joining us today and I wish you have a nice day ahead.

@turquoiseyez-yah 

You may also explore this connector

https://www.integromat.com/?pc=quickbooks