How to "Include in fixed line" on Service Quotation?

Hello community,

I would like to show only a global price of all the resources and material of a work task on a service quotation. I belive this should be possible as there is a Field “Include in Fixed Line” (see below) but I do not know how to set it to Yes as it is not editable on the Other demabds tab.

How can I do that?

Thank you !