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You can use online websites or applications to put the text (private Key) and then generate the QR. This is a sample of a website to generate the QR code.
http://www.qr-code-generator.com/ (Provided as sample)

https://4qrcode.com/ you can use the free QR Code generator online without paying anything is easy and fast it gives the ability to download the QR Code with multiple formats like PNG, SVG, PDF