On Apple iOS, there is a section in the settings where you can assign a letter sequence ("shortcut") to any text you want. When you type that letter sequence, the text you assigned to it shows up as the default replacement if you enter a space. Of course, there is another "replacement" available which is the letter sequence itself so that you can still have that letter sequence in your typing.
EXAMPLE: I assigned mmb to the address of my rented mailbox. When I type mmb on my iPhone, my rented address is the default and replaces the "mmb" I typed if I tap the space key. If I want it to say "mmb" then I have to tap the replacement option that has "mmb" in quotes.
In this way, I can send my rented mailbox address to anyone who needs it by typing mmb. There are several other short letter sequences that I also use on my iPhone. I'm switching to Android, so I'd really like this functionality on my Android.
I have a Samsung Galaxy 4 running Android 5.01. Apparently, there is no longer a "personal dictionary." I installed the "User Dictionary Manager" which allowed me to create a shortcut, but when I'm sending a message, the shortcut is ignored.