You can simply enter a new Donation in the portal field directly without a another layout.
Make sure the relationship table has "Allow creation of records" checked for the Donation't table of occurance.
Type new donations in a blank portal field.
PS: it is a bit more complicated to do as you want. Read about Go To Related Record script step.
Thank you for your response! Your solution solves my problem without the need to go to another layout!
Greetings from the Land Downunder!