I have a data entry form based on a SharePoint list.

'Staff Name' is a Combo box field (office365 users) and 'Issued to OR received from' is a text and a required field. Sometimes we have to issue material to the individuals who are not the organisational staff, hence, the reason for a separate text field.
What I would like to do is, if I select staff from 'Staff Name' field, it should copy the name to the 'Issued to OR received from' field. However, if I leave the 'Staff Name' field blank, I should be able to manually enter name of the individual in 'Issued to OR received from' field.
I have tried the Patch function in the OnChange property of 'Staff Name', while watching Shane Young video, unfortunately it did not work for me. Most probably I did not follow it properly.
Any assistance would be highly appreciated.