You could add this argument to the __init__ method or a custom class method of your model:
class MyModel(nn.Module):
    def __init__(self, user_input=''):
        super(MyModel, self).__init__()
        print('user_input is {}'.format(user_input))
        # create the modules here and register the hook
    
    def my_custom_method(self, user_input=''):
        print('user_input is now {}'.format(user_input))
        
    def forward(self, x):
        return x

model = MyModel(user_input='a')
model.my_custom_method(user_input='b')