This hard key is marked MEAS FUNC.
When you depress it, one of the menus below will open.
The current selection is indicated by text inversion that is also indicating the cursor position. Select the measurement function you want by depressing the corresponding softkey right below the display.
Alternatively, you can move the cursor to the wanted position with the RIGHT/LEFT arrow keys. Confirm by pressing ENTER.
A new menu will appear where the contents depend on the function. If you for instance have selected Frequency, you can then select between Frequency, Frequency Ratio and Frequency Burst. Finally you have to decide which input channel(s) to use.