Symbol not applying

[Velcra2077]

Hey guys,
so Ive read about people changing events color and symbol. I get the color done but the symbol doesnt work at all.
I tried with Iconify ( and the edited default class in calender config ) and I tried with FontAwesome, nothing seems to get a grip.

Also when using "ev.symbol" I get a TypeError, that ev.symbol.join is not correct ( whereever this "join" comes from" so somebody in the community mentioned to set "ev.symbol.push" - anyways, nothing applies a different/changed symbol

eventTransformer: (ev) => { if (ev.title.search("Geburtstag") > -1) { ev.symbol.push= "fa-solid fa-cake-candles"; ev.color= "#ff00ff"; } return ev ,}

Anybody can help?