you’re able to figure out and define the fields in QuickBooks Desktop (QBDT). For your future reference, you can also check this article: Use QuickBooks Desktop custom fields.
Resolution for 'Additional Customization issue: 'Custom Fields on Additional Customization' available: Yes.
The Additional Customization issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the Additional Customization issue yourself by looking for a resolution described in the article. If it is a complex Additional Customization issue or you are unable to solve the issue, you may contact us by clicking here or by using other Additional Customization support options.
For more information about resolving issues related to Additional Customization in Quickbooks/QBO or to get support for the issue 'Custom Fields on Additional Customization', click the link below: