Are Custom Fields Supported?
Yes – you can use the fmAccounting Link file to download the custom fields you have setup in QuickBooks Online by by clicking the Import Preferences button on the Preferences tab on the Company screen:
We hide/show the Custom Fields based on the Preferences that have been downloaded from QuickBooks Online:
Here's the sales Custom Field 1 showing on the Invoice Details layout:
N.B. if you make any changes to the settings for your Custom Fields make sure you download the Preferences again so the display options are correct in the fmAccounting Link file.
The QuickBooks Online API provides up to three custom fields for the following transaction types: Invoice, Sales Receipt, Estimate, Credit Memo, Refund Receipt, and Purchase Order. If you are using QuickBooks Online Advanced you cannot currently access the additional custom fields that you can create using QuickBooks Online Advanced. You can get more informaiton about custom fields and the QuickBooks Online API here:
https://help.developer.intuit.com/s/article/Enhanced-Custom-Fields-for-QuickBooks-Online-Advanced