I am postpaid customer and need to set up @txt.att.net and @mms.att.net aliases

I'm trying to block spam by creating aliases for [number]@txt.att.net and [number]@mms.att.net. It used to be at http://mymessages.wireless.att.com, but that is no longer available.

In a post regarding "Where did mymessages.wireless.att.com go?" ATT Help posted that it is now for postpaid customers only and is located at https://soc.att.com/2yYBA1w. It is no longer available; where is this feature now? I can't see it mentioned at all on soc.att.com.