Magic +1 Heavy Armor
 

I'm making a 5th Level Pathfinder Society character and can pick one 4th level item. How do I add +1 Magic Heavy Armor?

Never mind, found it.

Can you share? I can’t seem to find it.

Once you have purchased the item tap or click on it from the Play or Equipment pages. Scroll down and you will find a section for runes. Click or tap on the + in the upper right hand corner of the table. Find the potency rune you want and add it. The item has to be of Master Expert quality or higher to accept a potency rune.

Hi.
Rule book pg 370,Armor Potency is a level 3 magic item. A type +1 light or medium armor potency rune can be etched only on light or medium armor of expert or better quality.

or am I mistaken?

Nope, you aren't mistaken. I've corrected my post. Thanks!