Retrieve the selected value in field

Hello, I would like to create a custom menu in the "Employee Objectives and Activities" screen and I would like to retrieve the selected value of the "Performance period" field:

Indeed, in the "System info" tab, there are not many elements allowing me to retrieve this value ..:

In the PL / SQL code, I tried to fill in: &cmbPerformancePeriod, &labelPerformancePeriod ..
But it doesn't work, I get an error.

Is there a solution ?

Thanks in advance !