а так не работает?:
class WorkingDayForm(ModelForm):
def init(self, *args, **kwargs):
super(WorkingDayForm, self).init(*args, **kwargs)
self.fields['workman'].value = 'User Name'
class Meta:
model = WorkingDay
fields = ['workman', 'date', 'hours', 'hours_over_norm']
widgets = {'workman': PlainTextWidget}