how do i add a subject line on a text for Samsung Galaxy 6

[AC Question]

how do i add a subject line on a text for Samsung Galaxy 6

 

[hallux]

You don't. A text is not an email. Emails have subject lines, texts are just short messages - quick and to the point.

 

[JCToo]

Wrong hallux. Of course you can add a subject line to a SMS. Use the context menu (where there is an option to add one) or in Android 6 hold down on your name icon to the right of the message.
Very handy to have a subject when searching through lots of messages.....
And it is old school to limit an SMS to 160 characters....