what is the way to show my scripttag to my shopify store?

Question

I want to show an alert message to the customers that are landing on my shopify store. So I have written an endpoint to post a scripttag that is containing javascript code to show alert.

Here is api endpoint:

app.post("/api/script/add/script_tags.json", async (req, res) => {
  let status = 200;
  let error = null;

  try {
    console.log('script_taggggggggggggggggggggggggg',res.locals.shopify.session);
    const script_tag = new shopify.api.rest.ScriptTag({session: res.locals.shopify.session});
    script_tag.event = "onload";
    script_tag.src = "https://raw.githubusercontent.com/Sadiquzzaman/test/main/text.js";
    await script_tag.save({
    update: true,
  });
    res.status(200).send(script_tag); 
    

  } catch (e) {
    console.log(`Failed to add script tag: ${e.message}`);
    status = 500;
    error = e.message;
    res.status(200).send(e); 
  }
});

I have integrated this from my frontend. here it is:

const response = await fetch("/api/themes/update/"+id,
    {
      method: 'POST',
      headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({
        value: value
    
      })
    }
    );
    console.log(await response.json());

Here I can make sure that scripttag is adding successfully. Problem is it is not showing up to my store when I am viewing my store as a customer. As an add on: "https://raw.githubusercontent.com/Sadiquzzaman/test/main/text.js" this file contains code:

alert("Hello! Welcome to our shop.");