Where to put it depends on personal preference. On our demo board, I put it as a hold function on the Bank Down button.
In the editor, go to Buttons tab and double click the Bank Down button on page 1.
Set Button Mode to Hold
At the top of the screen, click Hold Functions
Set Button Type to IA
At the bottom of the screen, click Add CC Action
In the window that pops up, select H9 and Tuner. Click OK
Make sure that "Update on Preset Change" is turned on, and all other checkboxes are off
You can set the button's On Color setting to the color you'd like to see when the tuner is on. You can name the hold function as well.
Click Done, send the settings to your LT.
When you hold the Bank Down button for 2 seconds, it will turn on the H9 tuner (or turn it off if you had already turned it on).
You should also go into H9 settings and make sure the tuner function is set to CC19. There seems to be a lot of inconsistency between H9s where some have the CC numbers set from the factory, but others do not.