Swift 4 version of blyabtroi solution
let leadingMargin: CGFloat = 10
let trailingMargin: CGFloat = 10
let style = NSMutableParagraphStyle()
style.alignment = .justified
style.firstLineHeadIndent = leadingMargin
style.headIndent = leadingMargin
style.tailIndent = trailingMargin
label.attributedText = NSAttributedString(string: "Label with margins",
attributes: [NSAttributedStringKey.paragraphStyle: style])