Knowing the ins and outs of a job's complete kit of attacks and abilities is vital for succeeding in Final Fantasy XIV. The journey to mastering said job, whether playing PvE content or participating in PvP, starts not only with learning the effects of your abilities, but also deciding and memorizing their place on the hotbars.
Adding abilities to your hotbar for PvE content is relatively simple, as all it requires is finding an ability within the Actions and Traits menu and dragging it to an empty slot on your hotbar or using the submenu to add it to an accessible button on your controller. Adding those same abilities to your hotbar during PvP will usually lead to a not-so-fun time since any PvE skills are unavailable during battles against your fellow Warriors of Light.

How to add PvP abilities to a hotbar in FFXIV
PvP abilities, as the name implies, are unique to the three PvP modes in FFXIV, but you won't find them listed among your usual skills and attacks. While your hotbars in PvP areas will keep the layout and keybinds used in PvE, the bars you use in PvP areas are technically completely different hotbars.
When your PvP hotbars are active within an instance or zone, you can find the required abilities in the PvP Profile within the Character section of the Main Menu. From there, the Job section under the PvP Actions category holds all the abilities you'll use during any match. The names and icons of these abilities will likely be familiar to you if you're well acquainted with your job, but these PvP skills can differ significantly from their PvE counterparts, so make sure you read your tooltips.

Rather than trying to edit the layout of your abilities in the heat of battle during a match of Crystalline Conflict or Frontline, the best place to tinker with your PvP UI is at The Wolves' Den Pier, unlocked through the quest "A Pup No Longer" at your Grand Company's headquarters. When arriving at the Wolves' Den, the UI will indicate that your hotbars have been swapped to their PvP counterparts. Like your PvE abilities, all that's required from this point is to drag any skill within the PvP Actions menu to anywhere on the hotbar.

From that point forward, your hotbars will automatically swap to PvP Mode when entering the relevant areas and revert back once you leave. Once your PvP hotbars are set up for a job, you won't have to worry about manually swapping abilities around before a match, barring any additions or changes to the job in an upcoming update.