how to add email subject line

I'm gathering you're referring to email as opposed to an texting application/program. Email clients use a standard protocol to forward or reply a message.

When you forward an email the SUBJECT LINE is automatically added under the recipient's name with a "Fw:" moniker. One can edit the SUBJECT LINE to either remove the "Fw:" and/or simply the original subject and add a title to the subject.

Below the SUBJECT LINE in the the body of the message to be forwarded you'll see the original sender' and recipient' information with the date and time the forwarding message was originally received.

It's usually blocked with a dash lines above and below signifying to the NEW RECIPIENT the message they're receiving was forwarded.

You can remove the impertinent and intrusive information so the new recipient receives only the body of the text with any attachments you may have originally received.