You can enter your intelligence modifier as a bonus for your dex saving throws via the save options in the settings tab of your sheet (cogwheel on the top right, the saving throw boni are right in the center box, see screenshot ;) ), but you can only enter the number itself (i.e. 5 if you have a +5 int modifier/20 int score) and not its variable ( @{intelligence_mod} ), and thus whenever your intelligence changes, such as due to an ASI, you have to update that number. The alternative is a global save modifier (can be activated in the top right box), for which you can use the variable @{intelligence_mod}
rather than the actual number as the bonus, so that it automatically updates whenever your intelligence changes, but then you would have to
tick and un-tick that global save modifier depending on the kind of
saving throw you want to roll, as the bonus only applies to dex saves. That's also how I do my concentration saves for my bladesinger, who can add their int modifier bladesong is active; unfortunately it looks kinda odd, because it goes over the border of the field, Roll20 needs to fix something there; but with a little bit of custom CSS via Stylus it should be fixable :)