If the sender is determining the amount being sent, all they have to do is specify an amount and scan your public key's QR code, as displayed on the main Jaxx screen. Alternately, they can use the address displayed on your Jaxx screen (under "Your Current [Currency] Address")
Jaxx also enables you to generate a Payment Request QR code to request a specific amount of cash or bitcoin/ether. To do so:
1. Select Receive at the top left of the main Jaxx display.
2. Select whether you want the amount to be in the currency type or the default fiat currency of your choice.
3. Enter the requested amount into the text field.
4. Select Generate.