For SubScript use value for kCTSuperscriptAttributeName as @-1.
As per the document
@discussion Value must be a CFNumberRef. Default is int value 0. If
supported by the specified font, a value of 1 enables superscripting
and a value of -1 enables subscripting.
extern const CFStringRef kCTSuperscriptAttributeName
CT_AVAILABLE(10_5, 3_2);
Example- [lblHeader setText:@“Headers [Alpha1 – text”];
NSMutableAttributedString *headerSubscript = [[NSMutableAttributedString alloc]initWithAttributedString: lblHeader.attributedText];
[headerSubscript addAttribute:(NSString *)kCTSuperscriptAttributeName value:@-1 range:NSMakeRange(14,1)];
[lblHeader setAttributedText:headerSubscript];