Add data to Django form before it is saved?

In this simplified use case, I have a Django form that contains MyField_1 and MyField_2. The user fills out MyField_1. Before the form is saved as a model, I want the computer to add MyField_2 programmatically which is some data pulled in from another data source. The view I currently have looks like this:

class MyCreate(CreateView):
    model = MyModel
    fields = ['MyField_1']

How can I add the data to MyField_2 before the model is saved?