You can only activate the Bouwpas app with a QR code if you are on the construction site.
The foreman/'uitvoerder' who checks your ID is the only person who can activate the Bouwpas app, together with you.
The foreman/'uitvoerder' searches for your name in the Bouwpas system, on their own PC. On the screen, a QR code appears. Scan that QR code with your app.