A loyalty programme that only appears after the sale is a thank-you note. It is pleasant and it changes nothing.
The moment worth influencing is the one where the customer is looking at their basket deciding whether that is everything.
In the cart and at checkout
The order shows what it will earn. A real number, calculated on the actual contents, including any multiplier or bonus event that applies.
Two things happen with that number in front of a customer. Some of them add another item, because the earning is visible and increasing it is easy. And all of them are reminded that ordering from you accrues something, which is the fact the programme depends on and the one customers most reliably forget.
Bonus events say so
When a promotion is running, the estimate reflects it and explains why. "You’ll earn 680 points, double points this weekend."
A promotion the customer only discovers in the confirmation email did not influence anything. Surfacing it at checkout is the whole difference between a bonus event and an unrequested gift.
The guest nudge
This is the highest-value prompt in the plugin.
Someone checking out as a guest sees what they would have earned with an account. Not a newsletter box, not a vague suggestion to register: a specific number attached to the order they are placing right now.
That is a considerably better argument for registering than anything else available to you. The customer is mid-purchase, the benefit is concrete, and the cost is one field. Combine it with a signup bonus and the case is close to unarguable.
Converting guests into account holders is worth more than the points involved, because an account holder is a customer you can reach again, whose history you can see, and who can accumulate towards a tier.
Where it appears
The cart and checkout automatically. There is a shortcode too, for putting an earning estimate somewhere your theme or builder wants it instead.
Kept quiet when it should be
Products you have excluded from earning do not contribute to the estimate, so the number shown is what the customer will actually receive. An estimate that overstates and then under-delivers generates exactly the sort of support conversation a loyalty programme is supposed to avoid.