Why does the "Relate To" field not show for Calls Pop Up Search?

Using SugarCRM Professional 7

I went to Studio>Calls>Layouts and added the "Relate To" field to all of the possible searches but this field is still not showing when I open up an Opportunity, scroll down to the Calls subpanel, click "link existing record", and try to filter by the "Relate To" field.

Why is it not showing up? This is the process I use for every other field to show in this view but it is not working for this field.

Thanks in advance.