RAW, Holy Water requires an action to use.
The description for Holy Water says:
As an action, you can splash the contents of this flask onto a creature within 5 feet of you or throw it up to 20 feet, shattering it on impact. In either case, make a ranged attack against a target creature, treating the holy water as an improvised weapon. If the target is a fiend or undead, it takes 2d6 radiant damage.
Using the holy water in the way described here requires you use a full action. If you take the attack action, then technically this is a different action than the action described in the holy water description.
The order you do things should only matter when the rules specify that order matters.
Allowing you to use holy water as part of an attack action makes the sequence order dependent. Under this rule, if I throw holy water first, I can make no more attacks, but if I attack first, I can still throw holy water.
To be clear, my understanding of the D&D 5e action economy is that most things or order independent unless they specifically lay out the ordered conditions – this is not a rule, but a perspective on the rules in general. This is why I would stick with the RAW ruling I explained above.