I want to have the first line in an NSAttributedString for a UITextView indented from the right side on the first line.
So the firstLineH
The Setting Text Margins article from the Text System User Interface Layer Programming Guide has this figure:

As you can see, there's no built-in mechanism to have a first line tail indent.
However, NSTextContainer has a property exclusionPaths which represents parts of its rectangular area from which text should be excluded. So, you could add a path for the upper-right corner to prevent text from going there.
UIBezierPath* path = /* compute path for upper-right portion that you want to exclude */;
NSMutableArray* paths = [textView.textContainer.exclusionPaths mutableCopy];
[paths addObject:path];
textView.textContainer.exclusionPaths = paths;