Find Asp.net Web Forms control in script in Js (JavaScript) file

I have the following script in a Web Forms project:

function myScriptInAspxFile() {
         var obj = document.getElementById("<%= TextBox1.ClientID %>");
         if (obj) {                 
             obj.value = "My Text";                 
         }
     }

this works fine when I put the script inside the Web forms aspx file, but when I change the script to a Js (JavaScript) file the line:

var obj = document.getElementById("<%= TextBox1.ClientID %>");

The assignment returns me a null value for the obj variable. How could I find a control from a script in a Js (JavaScript) file? Greetings and thank you in advance.