how to access methods of content script in appended js file?

Question

content script file:

chrome.extension.sendMessage({ greetings: "hello from new page" }, function (response) {
    console.log("response", response)
    let storeData = response.store
    var readyStateCheckInterval = setInterval(function () {
        if (document.readyState === "complete") {
            clearInterval(readyStateCheckInterval);
            let th = document.getElementsByTagName('body')[0];
            let injectSc = document.createElement('script');
            let filePath = chrome.extension.getURL('src/inject/injectScript.js')
            // check window log in filePath 
            injectSc.setAttribute('type', 'text/javascript'); 
            injectSc.setAttribute('src', filePath);
            th.appendChild(injectSc);
        }
    },100)
})

function sendTokens(user_token){
    chrome.extension.sendMessage(user_token, function (response) {})
}

here in content script i was not able to get window object of the webPage (this is the limitation of content script , (https://developer.chrome.com/extensions/content_scripts) you cannot Use variables or functions defined by web pages or by other content scripts.) so i added one script tag in webpage and got window object access in injectScript file but now how do i use any method of content script or chrome.runtime.sendmessage form injectScript file. injectScript file

if (document.readyState === "complete" || document.readyState === "interactive"){
    console.log("window from inject script",window);
        let yajRex =  new RegExp('example.xyz.com')
        if(yajRex.test(window.location.href)){
           let user_tokens= {
               token1:window.paramValue,
               token2:window.paramValue
           }
        console.log(user_tokens);

    //    sendTokens(user_tokens)

    //getting undefined function error here
}}