To make it clear, I’m not at my computer right now to double check.
But, meanwhile at work, I was thinking “maybe I could macro Aimed Shot with Hunter’s mark on the same button”
I do not use Hunter’s Mark at all TBH. Experienced hunters feel free to advice or reprimand me on why should I use it.
BUT, I currently have a macro applying True Shot on CD, activating my trinket on CD, and firing Aimed Shot.
Could be possible to also add Hunter’s Mark? 
Last I checked, Hunter’s Mark was still on the GCD.
Although I admit that I haven’t looked that much since they took away the damage component.
Hunter’s Mark is on the GCD and has a Cooldown. You can macro it to Pet Attack but that is about the only recommended macro situation. Hunter Mark’s use now is purely cosmetic and anti-stealth.
Source: https://www.wowhead.com/news=318382/shadowlands-ptr-class-and-spell-hotfixes-for-oct-6th-hunters-mark-damage-compone
Blizzard has pushed hotfixes to the Shadowlands PTR which includes the removal of the damage component to Hunter’s Mark and additional Shadow Priest changes.
Hunter’s Mark - Apply Hunter’s Mark to the target,
increasing all damage you deal to the marked target by 5%. The target cancausing the target to always be seen and tracked by the Hunter.
Only one Hunter’s Mark can be applied at a time.
The cooldown of Hunter’s Mark is reset if the target dies.
Cooldown changed from 20 sec cooldown to None
No more cooldown either.
Or have they changed that again? 
The CD might be gone. I really haven’t spammed it to try it but the GCD will break it being macro’d into a damage dealing ability.
That it will yes.
It won’t macro I’ve tried it. Makes no sense since it has no damage and a wasted cool down to set it… why does it exist again? Deleting from primary hotbars and relegating to “when I need to tell people what to attack” use once a month button.
Its on the gcd so that when you get the anima power in torghast it isnt OP./s
It exists to mark Rogues and Druids in PvP situations. It is also very useful for the Pandaria challenge tames. While it doesn’t matter so much in these later expansions, past expansions have had occasional stealth enemies. Would rather have kept the damage component or at least have had the damage increase changed to raidwide, similar to how it used to be.