How to select a django-treebeard node in a form

I have created my tree structure, now I am trying to create a form that selects a node which I will use to do some operations.

When I render the form 

class SelectNodeForm(MoveNodeForm):

    class Meta:

        model = Category

with {{form.as_p}}

I get the fields

Path

Depth

Numchild

Value

Position

Relative to

I would like the form to just contain relative to (but allow multiple selection)

Looking at the code here it looks like that field is called 

https://github.com/tabo/django-treebeard/blob/master/treebeard/forms.py

_ref_node_id = forms.TypedChoiceField(required=False,
                                          coerce=int,
                                          label=_("Relative to"))

But I cannot render a field that starts with an underscore. "Variables and attributes may not begin with underscores: "

So how can I select a django-treebeard node in a form?

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