Ez, if you're doing PB Fitness, the plank is an abs+ workout!
But pushups, squats, and pullups all also work the abs. (If you don't think the other exercises work the abs, try doing them when your abs are sore!) And if you're looking for adding in an arms workout, that's what the pushups and pullups do.
Keep in mind that you have to be pretty low bodyfat % to have visible abs (for men, usually ~10% or less), and genetics also plays a part.
I'd say, just keep with the PBF plan, and when you get to a low enough bodyfat % to start seeing your abs, then you can decide if you want to do more!
If you are really determined to do more for your abs now, try hanging from the pullup position and draw your knees up to your elbows (repeatedly, like doing pullups). If you can't get your knees to your elbows, just draw them up as far as you can. A few sets of these will make your abs sore, and then you can see how the other exercises also use abs!
You make your choices, and you live with them. In the end, you are those choices.
"Strength is the mental and physical fortitude to endure, resilience to bounce back, and force to create change, allowing you to thrive in any circumstance and through any adversity." TrPAssassin