Olá, @dbuachalla. It would also be correct to use the future tense directly, but we usually don’t, because we’re used to using present-tense conjugations with a future outlook. It’s simpler and the message is well understood anyway. This is discussed in this Learning Note: Talking About the Future in Portuguese
So, this is the most common option:
Eu digo-te quando eu quiser ir. (present tense)
These are also correct, but less common:
Eu vou dizer-lhe/te quando eu quiser ir. (compound future tense)
Eu dir-te-ei/dir-lhe-ei quando eu quiser ir. (simple future tense - extremely formal/stuffy)