Add Smartwatch to my ATT prepaid plan

Zero $ because you cannot add a smartwatch to AT&T prepaid.  So any Smartwatch you purchase will be a Bluetooth device with your AT&T prepaid phone and service. 

None of the major carriers ( not at&t, not verizon, and not T-Mobile) allow smart watches on their prepaid service. There are a couple of mvnos that do.  Visible does.

In order to connect the SmartWatch with your phone to spoof your phone's number for call and text while the devices are separated or even while the phone is off, it requires a special software program. T-Mobile calls it digits, Verizon calls it number share, AT&T calls it number sync. These software programs only work for postpaid. 

This is either wrong info or has changed.  I just took the ATT prepaid SIM out of my phone, tested it on the smart watch I just bought my son, and it worked fine.

Which smartwatch?

Yeah which smart watch as ATT only supports googles, apple, and Samsung directly.  Your paying full price not the price of a watch line also.

And it should be pointed out that the watch is a standalone. It's not being used as a companion device to a phone.