How to set TextBox Value to the HiddenField inside FormView in ASP.Net

I am working on an asp.net c# WebForms application. The form has two controls, a GridView and a FormView. The FormView is populated when a row is selected in the GridView. This all works as expected. The problem is that I need the value of the Id field from the GridView to be the value of a field in the FormView's InsertItemTemplate. The following passes the Id of the GridView (venId) to the HiddenField on SelectedIndexChanged:

protected void rg_vendors_SelectedIndexChanged(object sender, EventArgs e)
    {
        foreach (GridDataItem item in rg_vendors.Items)
        {
            hdn_ven_id.Value = item["venId"].Text;
        }
    }

Next I would like to set the value of the venIdFk field in the FormViewto be equal to the value of hdn_ven_id when inserting the FormView data into the database. I don't know how to get this done, but here is an idea of what I need:

protected void fv_vc_ItemInsert(object sender, FormViewInsertEventArgs e)
    {
        (TextBox)fv_vc.FindControl("venIdFk") = hdn_ven_id.Value;
    }

The formview is fv_vc

The gridview is rg_vendors

The hidden field is hdn_ven_id

This is an asp.net c# webforms application

I am using VS2013