You can use <#> in your macro. Would have to change it each run, unless you party sort (/psort) before each run. Depending on your sort you would enter the number of the person you want in your macro.

Ex:

/micon "Cover"
/ac "Cover" <2>
/p Cover on <2>, stay close!!
/wait 11
/p Cover duration over. (or whateveryou want to put here)

In this example, the healer is the second one on the party list (<2>), if they were 7th then you would use <7> instead. I hope that makes sense.

So to answer your question you can specify who you wish to use the ability on. The number within <> is more reliable and quicker than you typing their name in there. This is also super helpful as WHM when buffing the party at the start of the duty.

Hope it helps.

Syn