QB Issue Resolution:

For your Templates, you would not use a Custom Field added to the customer name at all. That is not what custom fields are for.

On the Template, in Layout Designer view, select to add a New Text box, and put the text for the label, then position it. You can even add a Border (properties) to the Text Box and select Bottom only:

Signed: _________________________________________

But this part is not possible in QB: “and a pen or just their finger on our ipad”

To get that, you would need to Save or Send their invoice as PDF, and open it in Adobe Reader to allow for Signatures on this image.

Resolution for Issue 'I need to add a place where customers can sign on the invoice how do I do that?' available: Yes (Solved).
Source: Intuit Community forum.

I need to add a place where customers can sign on the invoice how do I do that?: 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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