custom arguments in backend

Hello,
I want to add a custom argument to the init function. But couldn't find instructions on how to pass the argument in the instantiation of the model.
Here is the code which has the basic init shown and custom argument.

`
class RNN_Model(LabelStudioMLBase):

def __init__(self, arg_1 = None,**kwargs):
`

Could someone point me to the instantiation of the model where I can pass custom arguments?