On every table, not just some

Print it small, on the same card as the table number or the menu, so it's part of what a guest already looks at rather than an extra thing to notice. A code on half the tables gets used by half the guest list.

At the entrance, big

One large sign near the guest book or the seating chart, somewhere everyone passes in the first five minutes, catches the people who arrive already reaching for their phone to photograph the venue.

Near the bar

The bar line is where phones are already out and where people stand still long enough to actually scan something. It's one of the highest-traffic, lowest-effort spots at the whole reception.

In the digital invitation

For guests traveling in, especially out-of-town family, put the link in the digital invite or the wedding website days before the event. They save it before they've even packed, and don't need to hunt for a physical card once they arrive.

What to skip

Don't rely on one code at the gift table alone, and don't assume the DJ announcing it once is enough. Redundant, low-effort placement beats one clever spot every time.