Run a command and email the output, with a subject, using msmtp

I have OpenWRT router, installed msmtp. I would like to write a command line (or a .sh file) that would send me the current public IP address, and let cron execute it every 15 Min. I found the posting: https://unix.stackexchange.com/questions/537082/run-a-command-and-email-the-output and tried to do the same with msmtp, and it did work: I did the following kdig +short myip.opendns.com @resolver1.opendns.com | msmtp xxxxxxxx@gmail.com and got an email with the current public IP address, but without a subject, of course, because I didn't set anything. Now, for msmtp, the way to put a subject is echo "Subject: Current Public IP" | msmtp xxxxxxx@gmail.com But I have no idea how I could combine these two things.... I would appreciate your help very much !