To enable single-key shortcuts, open the Preferences dialog box (Edit > Preferences), and under General, select the Use Single-Key Accelerators To Access Tools option.
After making changes in preferences you can use Shift+U key for highlighting.
For more information please refer to the help document- Keyboard shortcuts for Adobe Acrobat DC
Hope this helps.
thank God I found this. I highlight lectures all day (medical school) I spend so much time switching between text inserts and different highlight colors.... the frustration that their adobe didn't have hotkeys was eating at me... turns out the hotkeys were there, I just needed to activate them