How can I define imagekit spec from form fields in Django 2.0

[Jacques Rousseauu]

How can provide width and height dynamically from form field:

class PostForm(forms.ModelForm):
    image = ProcessedImageField(spec_id='posts:create:post_image',
                                processors=[ResizeToFill(width=600, height=400)],
                                format='JPEG',
                                options={'quality': 80})
    width_field = forms.IntegerField()
    height_field = forms.IntegerField()