Trying to drill into a outside 90 degree corner, 45° to each side, is tough, the drill bit keeps wanting to walk off.
You can whack a small flat on the corner.
But here is a way I did it perfectly. Using this corner drill jig

Using the drilling jig, One hand is enough to hold the jig while drilling. The small board touching the top of the jig is my measurement stick. I use that to set the precise reproducible location of the hole relative to the top of the workpiece.



I was able to attach this brace block to the inside. I prepared it by cutting off the tip and drilled a pilot hole in the tip on the drill press prior to assembly. The pilot hole location in the center of the brace and the precise location of the hole puts the brace at a precise location, too.


Another variation, same method, into corner of 4x4 post to place a Hanger screw-bolt.