C# How to call a method (.cs) from a function (aspx)

How do you call a method  that is in the aspx.cs program from a function in the aspx?
Inside my aspx page, I have a function, like shown below, that is called when the checkbox control is selected.  How can I call a method in my aspx.cs page in this function?  I have a method called  “protected void GetDataFilters(object sender, EventArgs e)” that I would like to perform when this checkbox is selected in addition to the script.  

<input type="checkbox" runat="server" id="supplierid" value="" onchange="havesupplier()">Have supplier?

        function havesupplier() {
            if (document.getElementById('ContentPlaceHolder1_supplierid').checked) {
            else {
                document.getElementById('ContentPlaceHolder1_txtsupplierID').value = "";

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David Johnson, CD, MVPOwnerCommented:
wrap your class in a namspace

namespace abc
and in your webform code behind add
using  abc;

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