I'm not familiar with your horn but you should have switched power at a number of points on your horn system, into the relay, out on one of the relay terminals and at the horn, you need to get a volt meter and test for 12v power with the ignition turned on.
One test lead on the positive terminal of the horn or relay and the other on the negative terminal of the battery, ensure the power drops off when the ignition is turned off.
Most horns have push on terminals and are a great place to get switched power as you can just use a piggyback terminal so you don't need to splice into any wires, as it is for a USB the loads can only be small so it will be OK, depending on the terminals used you could use the switched wires at the relay ?
You will still need to find a negative power source for the USB, that could be at the relay or at the battery.
Some of the 19 rides in my shed
2014 V2, Snoopy
2009 Wee, Pumbaa the pig
WR450F, The Blue Postie Bike
YZ250N smoker with rego, Stinky.
Yamaha MT09 (FZ09), The Scud missile.
Club Lead not Club Med.
He with the most toys wins.
Out of my depth in a puddle.
Live life on the edge you will see more that way.
Ridding a motorcycle keeps things in balance.
At the end of each trail and at the end of each day history is made.