How to pass codebehind information into javascript function

I have a button in my updatepanel and I'm trying to call a javascript function (after it does it's processing).

I put this code inside the Page_Load:

    Dim cstype As Type = Me.GetType()
    Dim cs As ClientScriptManager = Page.ClientScript
    cs.RegisterStartupScript(cstype, "modalScript", "jsModal('" + msg + "');", True)
    keywordSearch.Attributes.Add("onclick", "jsModal('" + msg + "');")

Client Side javascript:

function showModal(msg) {
    $("#modal-content, #modal-background, #modal-close").toggleClass("active");
    var returnString = msg;

    $('#modal-content div').each(function (index) {
        $(this).html(returnString);
    });
}

How do I pass the values I gather from the server side click event into the javascript function?