ZDDP is a zinc additive specifically added for flat tappet camshafts. It is to specifically lubricate where the lifter rides on the cam lobe but it helps where the lifter rides in the lifter bore of the block as well.
Read your cam instruction sheet about the minimum amount that should be in the engine oil for the camshaft.
The quantities are also listed on the contents label of the oil.
You can buy it separately to add to the oil but the "break in oil" generally contains sufficient amounts.
Read your content labels.
With a roller lifter camshaft ZDDP content is really a minor issue as the point of the roller lifter design is that the lifter rolls on the cam lobe rather then slides on in. That reduces, but doesn't totally eliminate friction, and it permits the cam lobe to have a steeper ramp that the roller lifter can follow whereas the flat lifter cannot follow a ramp that steep.
So therefore the cam lobe can be ground to improve the performance with a smoother idle then can be obtained with a flat lifter design.