Stock Samsung message app Tab S 10.5 ?

If you mean the texting application, you can't. There's no texting available to tablets.
 
Yes, that is what I'm talking about. I have the Verizon message app that works on the Tab S but wanted to find the regular old Samsung one.
 
Not available for Tablets. Verizon does have a messaging app that you can use on different devices, but that's because it's a service they offer (just as if you were using Google Voice for texting you could use Hangouts or the Google Voice app).