I have created a custom field as advised in help this now needs to accept the relevant information from our MIS how can an API populate a custom filed I have created?

We have a separate and dedicated forum that handles API concerns for QuickBooks Online (QBO), Lindar.  

API concerns are managed by our team of Intuit Developers, focusing on addressing programming and system-related issues that business people like you may encounter.

I suggest creating an account and posting your question from there. To ensure you receive a suitable response regarding populating your API to a custom field from MIS to the PO field.

1. Go to this link: https://help.developer.intuit.com/s/.
2. Choose QuickBooks Online under What do you need help with?
3. Tick Ask a Question.
4. Log into your existing Intuit Developer Account or create a new one.
5. Enter your concern.
6. Click Ask.

You can also check some helpful API topics on the Intuit Developer page by going to the API Docs & Tools menu, selecting QuickBooks Online, and then clicking API Explorer.

Refer to this article if you plan to run reports to track your sales.: Run reports in QuickBooks Online.

Keep us updated if you have other concerns about QuickBooks Purchase Order concerns. We'll do our best to respond promptly. Take care.