How to have a constant variable just inside a custom event and not add it as variable of blueprint

[Nawrot]

I just want to add, that such solutions are what i would call “a hack”.
There is risk that they (Epic) change such blueprint behavior a bit in future (for eg. new optimization, or merging stuff with Verse). And this method will not work. In a year or two your game prototype could stop working, and You may forget about this nice hack.
So imo. it is better to think what you want to do, why you need it, and what is “unhacky” solution. Less hassle with maintaining code later.