django ModelMultipleChoiceField set initial values

I have the following code:

category = forms.ModelMultipleChoiceField(
    label="Category",
    queryset=Category.objects.order_by('name'),
    widget=forms.Select(
        attrs={
            'placeholder': 'Product Category', 'class': 'form-control'}),
    required=True
)

how do I set an initial value in the select box like "Choose a category" so that the select box should have a list of categories with the initial value being "Choose a category"

If you pass a QuerySet object as an initial value and if widget=forms.CheckboxSelectMultiple, then the check boxes are not checked. I had to convert the QuerySet object to a list, and then the check boxes were checked:

YourForm(
    initial={
        "multi_field": [
            cat for cat in Category.objects.all().values_list("id", flat=True)
        ]
    }
)