How do I access a <div> attribute from the C# code behind?

in my asp page from the C# code behind. While I can get at it by ID in the main Page_Load section, I cannot access it from a function on the page, which is where I want to do the work.

My div:

<div id="searchParams" modifiedstring="This is a Test" runat="server"></div>

In the C# code-behind I am trying to access it from this function:

public static void getItemsBySubject(string search)
{
    List<HeritageItems> ItemList = DataAccess.FindItemsBySubject(search);
    string modifiedstring = Jsonify(ItemList);
    *I Want to access searchParams here but cannot*
}

I get an "object reference is required for the non-static field, method, or property" error

However I can access it just fine here on the same code behind page:

protected void Page_Load(object sender, EventArgs e)
{
    searchParams.etcetera
}

What I have tried:

I'm obviously a beginner in this so I could be missing something very obvious. All the help pages I've seen online for similar questions show the example in the Page_Load function. Which makes me think that has something to do with it....

But I suspect (hope) there is a way of accessing this from another stand alone function that I will call from another page. When working in console apps this might require an object to be instantiated, but my attempts at trying a few different methods haven't worked because the