reverse m2m widget in django admin

admin.py

from django import forms
from django.contrib import admin
from django.db import models
from django.contrib.admin import widgets
from django.contrib.auth.forms import ReadOnlyPasswordHashField
from django.utils.translation import gettext as _

class CustomUserForm(forms.ModelForm):
    company_structures = forms.ModelMultipleChoiceField(
        queryset=CompanyStructure.objects.all(),
        widget=widgets.RelatedFieldWidgetWrapper(
            widget=widgets.FilteredSelectMultiple('company structures', False),
            rel=models.ManyToManyRel(
                field=CustomUser,
                to=CompanyStructure,
                through=CustomUser.company_structures.through
            ),
            admin_site=admin.site
        )
    )

    password = ReadOnlyPasswordHashField(
        label=_("Password"),
        help_text=_(
            "Raw passwords are not stored, so there is no way to see "
            "the user’s password."
        ),
    )

    class Meta:
        model = CustomUser
        fields = '__all__'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        if self.instance and self.instance.pk:
            self.fields['company_structures'].initial = self.instance.company_structures.all()

        password = self.fields.get("password")
        if password:
            if self.instance and not self.instance.has_usable_password():
                password.help_text = _(
                    "Enable password-based authentication for this user by setting a "
                    "password."
                )
        user_permissions = self.fields.get("user_permissions")
        if user_permissions:
            user_permissions.queryset = user_permissions.queryset.select_related(
                "content_type"
            )

    def save(self, commit=True):
        instance = super().save(commit=False)
        if commit:
            instance.save()
        # Save the ManyToMany relations
        instance.company_structures.set(self.cleaned_data['company_structures'])
        return instance

models.py

from django.contrib.auth.models import AbstractUser
from django.db import models


class CustomUser(AbstractUser):
    objects = CustomUserManager()

    def __str__(self):
        return self.username

class CompanyStructure(models.Model):
    name = models.CharField(max_length=255)
    users = models.ManyToManyField(
        CustomUser, 
        related_name='company_structures', 
        blank=True
    )
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    def __str__(self):
        return self.name