I don't use the samsung keyboard but when Iooked in the settings I saw no option to create shortcuts or for a personal dictionary...if you install a different keyboard like google keyboard, swype, slide it etc they le tyou save word and shortcuts...for example I use swype and I have added ur to ditionary so when I slide or tap ur it comes up in suggestion bar...you can also have it so ur brings up your in suggestion bar by making it a short cut word...so you can have both...I just don't see it available in native keyboard.
Ther other keyboards will let you create shortcuts like gmail and it substitutes your whole gmail.com address etc.