Always use an apostrophe and an s to mark a possessive. Beware of the ambiguity created by the omission of both:
If the noun you wish to make a possessive already ends with an s, you have the option of adding just an apostrophe, or an apostrophe and an s:
But preferably re-write the statement to remove the awkward grouping of letters:
Just a reminder: its is the possessive form of it. No apostrophe is needed. It’s is a contracted form of it is. See section 1.4 for advice against using contractions.