[Answered ]-How to add a ModelMultipleChoiceField to a model?

Here is my solution:

class ProductsSetForm(forms.ModelForm):
   
    class Meta:
        model = Group
        exclude = []
 
        products = forms.ModelMultipleChoiceField(
         queryset=ProductsInstance.objects.all(),
         required=False,
         widget=FilteredSelectMultiple('products', False)
    )

    def __init__(self, *args, **kwargs):
        super(ProductsSetForm, self).__init__(*args, **kwargs)
        if self.instance.pk:
            self.fields['products'].initial = self.instance.products_set.all()

    def save_m2m(self):
        self.instance.products_set.set(self.cleaned_data['products'])

    def save(self, *args, **kwargs):
        self.total_amount = sum(product.price_for_client for product in self.products_set.all())
        instance = super(ProductsSetForm, self).save()
        self.save_m2m()
        return instance
    class ProductsSetAdmin(admin.ModelAdmin): 
        form = ProductsSetForm 
        list_display = ('name', 'borrower', 'created_date', 'status', 'total_amount')
        fields = ['id', 'name', 'products_set','products', 'created_date', 'borrower', 'status', 'total_amount']
        list_filter = ['borrower','created_date','status']  
    admin.site.register(ProductsSet, ProductsSetAdmin) 

You list the relation in the filter_horizontal attribute ^[Django-doc]:

@admin.register(ProductsSet)
class ProductsSetAdmin(admin.ModelAdmin):
    list_display = ('name', 'borrower', 'created_date', 'status', 'total_amount')
    fields = [
        'id',
        'name',
        'products_set',
        'created_date',
        'borrower',
        'status',
        'total_amount',
    ]
    filter_horizontal = ['products_set']
    list_filter = ['borrower', 'created_date', 'status']

You can also use filter_vertical attribute ^[Django-doc] where the two boxes appear above and below each other.