After you have created your form, you will be able to add a pre-filled amount to all of the possible form options (donation page, embedded form, popup form and donate button). To do so, please visit your Campaigns page while logged into your account, and click the '<> Embed Codes' button:
Once that's done, you will be able to add a pre-filled amount.
Embed Donation Form:
Popup Donation Form:
Donation Page Form: