The em-dash can be created on computer keyboard by using holding down the ALT key and typing 0151 on the numeric keypad with num lock on. It looks like this — as opposed to double hyphen -- that looks stupid. Total pain in the ass on smart phones.
Not worth the trouble, IMHO, unless you are sending something out for publication.
Actually, adding em-dash and other non-standard punctuation is not
that much of a pain in the ass on a smartphone. In some cases, simply holding down the hyphen key and selecting will do the job, but in those cases that it doesn't, switch to the numerical keys and press on the minus key. That typically generates a choice of three: hyphen, em-dash and en-dash. I really don't think the double hyphen looks all that bad, though, and gets the point across pretty easily.
Remember that the ALT codes only work on keyboards with a dedicated numerical keypad on the right, not the number keys along the top. This can be an issue with smaller laptops that don't have a separate numerical pad. In this case, Character Map (in all Windows systems) is another method that you can use to input non-standard characters. But pretty much any word processor worth its salt (Word, LibreOffice Writer, WordPerfect, etc.) will differentiate the hyphen and dash for you automatically.
