Il y aura un dress-code, je devrai porter un kilt.

You can use the present for a future event in French in about the same circumstances where you can do it in English. You need a future tense to refer to a specific future event, but if you’re talking about an event that could potentially recur, then you can refer to it in an “intemporal” way in the present tense.

There is a dress code, so I have to wear a kilt.
There is/will be a dress code, so I will have to wear a kilt.

Il y a un code vestimentaire, donc je dois mettre un kilt.
Il y a/aura un code vestimentaire, donc je devrai mettre un kilt.
Il y a un code vestimentaire, donc il faut que je mette un kilt.
Il y a/aura un code vestimentaire, donc il faudra que je mette un kilt.