How do I make a national spirit that makes the fighter production cost less?

If I put in the targeted modifier box:

fighter_equipment = {

build_cost_ic = -0.25 instant = yes

Would that work? 

I tried finding out the line of code using the code builder but I just copied and pasted a line of code from the national spirit code from Germany called "jaegernotprogramm"

What you have is correct. That would give just fighters a 25% reduction in cost (though you did miss the closing brace - maybe a copy/paste error)

fighter_equipment = {

build_cost_ic = -0.25 instant = yes

}

Oh yeah. I did miss the bracket but I added that later. Thanks!