To do that you would use URL parameters like this:

https://secure3.convio.net/SHORTNAME/site/Donation2?2200.donation=form1&df_id=2200&set.DonationLevel=3042&set.Value=500&set.OptionalRepeat=TRUE

Where &set.DonationLevel=3042 is the Donation level with the associated Level ID and &set.Value=500 is going to populate that field with $5.00 by default.
Using &set.OptionalRepeat=TRUE will automatically check the box that will make the donations repeat every month.

You can see all the options available here:

https://www.blackbaud.com/files/support/helpfiles/luminate-online/help/luminateonline.html#../Subsystems/donation_management/Content/Concepts/Admin_Online_Giving_Management_DonationForms_HiddenInputs.html

If you are noticing that the URL parameters are not populating, contact support and reference this Solution ID.

These values can not be used if the form is embeded into a pagebuilder page. As these URL encoded values are built into the Donation2 applet.