How to execute a function inside chrome.tabs.executeScript

Question

I have this code:

chrome.browserAction.onClicked.addListener(function(tab) {
chrome.tabs.query({ active: true, currentWindow: true }, tabs => {
       chrome.tabs.executeScript(
           tabs[0].id,
           {
              code:            // HERE I want much more space to write
              'var to_filter = document.querySelector("iframe").contentDocument.querySelector("body").querySelectorAll("tbody")[1].innerHTML; alert(to_filter);' 
           },  
            downloadFiles
          );
   });
});


function downloadFiles(resultsArray){
  
}

I have to execute a lot of code inside the code tag, and after that, I have to send a list of links to the downloadFiles function. The problem is that I cannot write all the code in the code field, I would like to use a function instead. How can I call a function to execute instead of writing only in that line?

------ Update

  chrome.browserAction.onClicked.addListener(function(tab) {
  chrome.tabs.query({ active: true, currentWindow: true }, tabs => {
       chrome.tabs.executeScript(
           tabs[0].id,
           {
              code:'(${ inContent })()'  // ERROR
              //'var to_filter = document.querySelector("iframe").contentDocument.querySelector("body").querySelectorAll("tbody")[1].innerHTML; alert(to_filter);'
            },
            downloadFiles
          );
          function inContent() {
            alert("test")
          }

   });
});

I get: VM138:1 Uncaught SyntaxError: Unexpected token '{' in the commented line above.