QB Issue Resolution:

QB works on customer names for look up, you can put the customer number in the name block with the name, that is as close as it gets

gear>feedback and report/suggest it to the developers

Resolution for Issue 'Is there a way to use customer numbers in QB Online?' available: Yes (Solved).
Source: Intuit Community forum.

Is there a way to use customer numbers in QB Online?: 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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