To do that you would use URL parameters like this:

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:

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.