Press Alt+F9 to show field codes (optional but helpful).
Insert the outer field braces with Ctrl+F9 (don’t type { } manually).
Use this field code (exactly):
{ DISPLAYBARCODE "{ MERGEFIELD Url }" QR \q 3 \s 120 }
\s 120 controls size (try 80–200).
\q 3 is error correction level (0–3).
