QB Issue Resolution:
Why not use the ref number field just above where you circled on the screen shot, that is what it is for
Resolution for Issue 'how to customize vendor bill forms. we need to a column for external document number for vendors' available: Yes (Solved).
Source: Intuit Community forum.
how to customize vendor bill forms. we need to a column for external document number for vendors: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.
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