Prefill MultipleChoiceField django

Lets say I have the following model

class Application(models.Model):
    occupation = models.TextField()

and form

class ApplicationForm(forms.ModelForm):
    def __init__(self, *args, **kwargs)
        super().__init__(*args, **kwargs)
        self.fields['occupation'] = forms.MultipleChoiceField(choices=OCCUPATION_CHOICES, widget=CheckboxSelectMultiple)

    class Meta:
        model = Application

When I use it with a instance (for example via admin) the choices are not selected.

What am I doing wrong?

Edit: Clarification: When I select some choices, I hit submit it saves the data. They look like ['undergraduate', 'postdoc'] in the database. But they are not checked in the form anymore.