Adding New Custom Field In Contract Tab !

Hi,

I am new to CLM!

I was trying to add new custom field to contract document, below Description and Contract document type

I went to extension Definition under configuration and tried “contract document type” and “contract document” in “Extended Class Name field “but could not add new field under description.

Can any one  please let me know what exactly should I use for "Extended Class Name" to add these new field.

Thanks,

Roopesh