Unfortunately, this is at best a two-button task. You can cycle up/down to soft target, then confirm (default:numpad0) to hard target. My original reply has been amended to include this detail.
Without a magic keyboard, the only way I can think of to make it a one-button task requires abusing the /hotbar command to switch out an entire bar just for one or two buttons, which most people will find quite cumbersome.
with eight bars that are identical except that one button is different and targets the next party member in sequence. Maybe have a second button that cycles in reverse.Code:/ta <2> /hotbar copy JOB BARNUM CLASS BARNUM
This approach is far from perfect, and I wouldn't really recommend it, as (1) it uses a whole lot of hotbars for minimal benefit and (2) isn't smart enough to understand light party v full party, so you'll need extra presses if you find yourself cycling the wrong direction in a light party.
In the end, it's best to get used to the quirks of the soft target, or use mouseovers, or click the party list. Basically, anything but that wonky hotabar macro scheme.