Be careful what substances you decide to create endogenously from the third eye. Some of these can kill you like opiates.
For this spell, begin research into the substance you want the third eye to produce. Meditate and imagine the pineal gland producing the substance. You can visualize the molecule, crystals (For DMT) mushroom spores (For Psilocybin) or anything else that represents the substance you want to be produced inside of your own body.
This is meant for microdosing or enhancing your spirituality and not getting high off of your own supply. You must also keep in mind that magically putting drugs in your bloodstream can cause you to fail a drug test.
For those who do not believe the third eye can inject drugs directly into the bloodstream, the third eye can still manifest things like a good drug dealer or finding mushrooms in the woods.
For the scientific thing that’s how things happen in a secular way. This can only be from the spiritual side.