To allow your users to register interest in a product that's out of stock, you'll need to provide them a form to enter their email. This will subscribe them to an email notification, so that they can be notified when the product is back in stock.
Check out our ready-to-go Tailwind template.
The form will need 3 fields. 2 hidden fields and 1 visible field where the user can enter their email.
<input type="hidden" name="action" value="craft-commerce-back-in-stock/base/register-interest">
<input type="hidden" name="variantId" value="{{ product.defaultVariant.id }}">
<input type="text" name="email" value="{{ currentUser.email }}">
You can also include an options value to save additional information with the form submission. This should be in the form of a JSON object. This input is entirely optional.
{% set options = { title: 'Some Title', productAttribute: 'Some Value' } %}
<input type="hidden" name="options" value="{{ options | json_encode }}">
Once the user fills in their email, they'll be notified when the product comes back into stock.
You can also set an email to be sent as soon as the user registers their interest, in the form of a confirmation email.
If privacy is a requirement you'll want to enable "Automatically Purge Notification Requests". This will delete the users information once they have been notified rather than being kept in the database.