Hi @jxmolina,
To do this, you can use any free QR code generator, for example https://www.qr-code-generator.com/.
First, if you don’t have it yet, you would create your landing page that features the form your contacts should fill out. Once you’ve published both, paste the link of the landing page into the QR code generator.
After generating the QR code, you can download it and add it to your post card template.
Let me know if you have any follow-up questions after reviewing the resources that I linked above.
Hope this helps!