Your link goes to a site which uses 'Asian English' (or, more specificaly, 'Indian English'), where many of the sentences given as examples will appear as incorrect to the (predominantly) British members of this site.
That's because Asian/Indian English uses some of the sentence structures of Hindi, where there's no definite article (and infrequent use of an indefinite article). So it's not a good starting point for those of us who use British English (where definite and indefinite articles are extremely important).
The British Council's website is a useful resource for learning (British) English. See the section relating to 'something that is fixed in the future', here, for a few examples of how the present tense can be used to refer to future events:
https://learnenglish.britishcouncil.org/en/english-grammar/verbs/present-tense/present-simple