Well, it seems it doesn't work for the custom battles(Custom/Quick) because they do not load scripts, but it works fine for the other modes(Historical/Campaign). I'll write a thorough guide later, but until then:

Add the two specified lines to the end of a descr_battle.txt or descr_strat.txt. If the file already contains a "script" part(historical maps are like that), change the last line to StartupScript.txt(ie: descr_script.txt => StartupScript.txt ). Also add the StartupScript.txt in the same folder. Those two steps must be repeated for each battle/campaign. Oh and don't forget to press the pause button(which is the P key) on your keyboard during a battle to activate it.