public class DocumentViewModel
{
    public int Id { get; set; }

    public int ActionId { get; set; }
}

<form....>
    <input id="docActionId" name="docActionId" asp-for="ActionId" type="hidden" value="initialValue" />
</form>

$("#save_btn").on("click", function ()
{
    $("#docActionId").val("test1");
});

$("#submit_btn").on("click", function ()
{
    $("#docActionId").val("test2");
});