display error message on front-end asp.net mvc

How do I display a message on a div <div asp-validation-summary="All" class="text-danger"></div> when I am sending a message from the back-end?

[HttpPost]
        public IActionResult Signup(Signup signup)
        {
                var result = DBGetCls.GetList("SELECT * FROM Usertable WHERE Email = '{0}' ", signup.Email);
                if (result.Count > 0)
                {
                    object notFound = new { message = "Email has been used!" };
                    return BadRequest(notFound);
                }

So my form would look like this:

<div id="signup">
    <form asp-action="Signup">

        <div asp-validation-summary="All" class="text-danger"></div>

        <div class="field-wrap">
            <label asp-for="FullName" class="control-label active highlight">Name :</label>
            <input asp-for="FullName" class="form-control" />
        </div>
         .
         .