SharePoint 2013 how to run JavaScript or jQuery for calendar add event popup window

Follow below steps:

1. Go to New form page of calendar list form using URL in below format:

   https://siteUrl/Lists/ListName/newform.aspx
   

2. Edit the page

3. Add Script editor/Content editor web part on page

4. Add below code in web part to auto-populate the Location field on form:

<script type="text/javascript"> 
    function autoPopulateLocation(){
        // your code
        document.querySelector("input[title^='Location']").value = "My custom value";
    }
    _spBodyOnLoadFunctionNames.push("autoPopulateLocation");

</script>

_spBodyOnLoadFunctionNames.push("autoPopulateLocation");

5. Save/Stop Editing the page.

Output:

Update from comments:

To disable the textbox, use below code:

document.querySelector("input[title^='Location']").disabled = "disabled";

OR with readonly attribute like:

document.querySelector("input[title^='Location']").readOnly = true;