How do I add a manytomany model field instance inside a ModelForm?

I have implemented a CustomUser model(using AbstractUser) and it has some ManytoManyFields. Now I can add an instance of it inside the admin panel but I haven't found out any ways to do it inside a ModelForm(without already having some ready made instances of it in database). The pic below shows the difference I'm trying to describe between what is available inside the admin and inside html templates form.

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