Hi,

First, inherit the 'pos.session' model in your python file. Define a new function called '_loader_params_your_model' that returns a modified version of the result from the parent function (replace your model with name of the model you want). To do this, first, call the parent function using the 'super' function and store the result in a variable. Then, use the 'extend' method of the 'fields' list to add new fields to the list. Finally, return the modified dictionary. Your function should look like this

    def _loader_params_res_partner(self):
        """
        This function loads the parameters of res.partner in the session.
        ----------------------------------------
        @param self: object pointer
        @return result: params of res.partner model
        """
        result = super(PosSession, self)._loader_params_res_partner()
        result['search_params']['fields'].append('your_field_name')
        return result

Hope it Helps