automatically post an e-mail subject line when calling a mailer

Use Swift to call a mailer in the app and
I wanted to automatically enter the destination email address and subject line when I called, so I organized the following programs:

// Generate mailer URL.
let myMailURL:NSURL = NSURL (string: "mailto:[email protected]")!
let myMailSub:NSURL=NSURL(string: "Subject=\(dateString)"!

UIAApplication.sharedApplication().openURL(myMailURL)//mail address
UIAApplication.sharedApplication().openURL(myMailSub)// Subject

There are no errors in simulators or debugs, but
If you check it on the actual machine, it will fall off as soon as you call the mailer.

In my estimation, I thought the subject was wrong, and I thought the subject was strange.

If you don't mind, please let me know.
Thank you for your cooperation.