How do I change the Price List field to no be mandatory filled on

Hi all

I am fairly new to Dynamics CRM so apologies for what may be a very simple solution.

I am trying to remove the red asterix (field must be filled in) from an Order form we use.

I have gone into Settings, Customization, Customize the System.

Then selected the Order form from Entites

I can see the Price List field and next to it is a Red lock and the Red asterix.

I double click on the field but I don't see any option to say that the field doesn't need to be filled in.

Can anyone point me in the right direction.

Thanks in advance

Steve