Hi,
In the Code behind pass the Client Id of the Texbox in your Javascript function as below:
textCustomerName.Attributes.Add(GetCustomers('"+textCustomerName.ClientID+"'))
and in the Javascipt function use that id.
var customerSubString = document.getElementById(parameterName).value;
Replace the 'parameterName' with the one put in your function.
Regards,
Ravi