Django admin mptt: how to show TreeNodeChoiceField instead of ModelChoiceField when add/change with FK

Question

I'm working on a Django app that includes some mptt based tree models.

I have these models (simplified):

class Asset_Type(MPTTModel):
    parent = TreeForeignKey('self', on_delete=models.CASCADE, blank=True, null=True, related_name='children', verbose_name='Parent')
    name = models.CharField(max_length=64, unique=True, verbose_name='Asset Type Name')

class Asset(models.Model):
    name = models.CharField(max_length=64, unique=True, verbose_name='Asset Name')
    type = models.ForeignKey(Asset_Type, on_delete=models.CASCADE, blank=False, null=False)

I have these admin classes declared (simplified):

class Asset_TypeAdmin(DraggableMPTTAdmin):
    mptt_level_indent = 20
admin.site.register(models.Asset_Type, Asset_TypeAdmin)

class AssetAdmin(admin.ModelAdmin):
    list_display = ['name','type']
    list_display_links = ['name']
admin.site.register(models.Asset, AssetAdmin)

Everything works almost perfect, except this:

When I add a new asset I get a template/form with a drop-down list where I can select the Asset_Type. The issue is that the drop-down list is of the class ModelChoiceField so it does not show the tree hierarchy. It shows this in the drop-down list:

---------
Root 1
Child 1.1
Child 1.1.1
Root 2
Child 2.1
Child 2.1.1

I would like it to be class TreeNodeChoiceField so it shows this:

Root 1
--- Child 1.1
------ Child 1.1.1
Root 2
--- Child 2.1
------ Child 2.1.1

I've found information related to this here: Django docs, but honestly I have no clue on how/where should I tell Django that the field to use in the add/change form should be TreeNodeChoiceField. I've successfully overridden the change_form.html template for the admin for the assets class of my app (creating ./project/app/templates/admin/app/asset/change_form.html) but I cannot find where to specify in that file (or anywhere else) what's the field to use.