How to stop the default value and blank out the quantity field in iSupplier portal ?

Hello,

While creating an invoice in iSupplier portal

iSupplier portal defaults the quantity left to be invoiced ( Available quantity) automatically to the quantity field on the invoice.

Our suppliers are submitting the invoices without checking this field and over invoicing us, which is creating rework.

We wish to blank out the quantity field in iSupplier portal, so that the supplier will key in the quantity manually.

I think this should be possible using form personalisation. If it is possible How ?

Regards,

Matilda