Can I enchant a necklace with the equivalent of a healing potion? (DnD 5e)

I know nothing of magic items really, especially custom ones, so I’m just going to describe an ideal scenario and I’d love some help in how something like it could be achieved, please! Am looking for some way to enchant a necklace with the ability to give some HP to the wearer. In an ideal world this would be triggered if the wearer hits 0HP, and would be rechargeable in some way. My player is not a spell caster, so I would be looking for a merchant or someone in town who could do this for her, if you have any advice on how much this might cost, up front and for each recharge or something. (For flavour reasons, she already has a specific necklace she’d like to enchant) If anything like this is possible, I’d love some help! Thanks!