Open the appropriate print template, e.g., Invoice, and at the desired location, e.g., at/under Border, insert the following placeholder:
<div>%piggy%</div>

Attention:
This will only generate the QR code.
Print code example with layout
This is how an example of the print code with layout could look like:
<div style="text-align:left;margin-left:60px;font-size:x-large;width:180px"><div style="text-align:center"><b>Scan me for Piggy points!</b></div></div>
<div style="text-align:left;margin-left:50px;">%piggy%</div>
<div style="text-align:left;margin-left:120px;">piggy.nl</div>
(The "Scan me for Piggy points!" can be replaced with any text).