Assign role for users during registration in Asp.net identity MVC

Question

I'm pretty new to asp.net identity and I have been reading some tutorials about it. They provide functions one by one but I haven't found something that guide me to do 2 functions at the same time. In details, I have a register form and I want to assign role for the user when they click submit.

• This is action which I assign user to role:

public ActionResult AssignUserToRole(string userId, string roleName)
{
    ApplicationUserManager UserManager = HttpContext.GetOwinContext().GetUserManager<ApplicationUserManager>();
    IdentityResult result = UserManager.AddToRole(userId, roleName);
    if (result.Succeeded)
    {
        //Whichever Page you want, you can redirect
        return RedirectToAction("Index", "Home");
    }
    foreach (string error in result.Errors)
    {
        ModelState.AddModelError("", error);
    }
    return View();
}

• then I have register form:

<h2>@ViewBag.Title</h2>
@using (Html.BeginForm("AddUser", "Home", FormMethod.Post, new { @class = "form-horizontal", role = "form" }))
{
    @Html.AntiForgeryToken()
    <h4>Create a new account.</h4>
    <hr />
    @Html.ValidationSummary("", new { @class = "text-danger" })
    <div class="form-group">
        @Html.LabelFor(m => m.UserName, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.TextBoxFor(m => m.UserName, new { @class = "form-control" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(m => m.FirstName, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.TextBoxFor(m => m.FirstName, new { @class = "form-control" })
        </div>
    </div>
    <div class="form-group">
        @Html.LabelFor(m => m.LastName, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.TextBoxFor(m => m.LastName, new { @class = "form-control" })
        </div>
    </div>
    <div class="form-group">
        @Html.LabelFor(m => m.Email, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.TextBoxFor(m => m.Email, new { @class = "form-control" })
        </div>
    </div>
    <div class="form-group">
        @Html.LabelFor(m => m.Password, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.PasswordFor(m => m.Password, new { @class = "form-control" })
        </div>
    </div>
    <div class="form-group">
        @Html.LabelFor(m => m.ConfirmPassword, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            @Html.PasswordFor(m => m.ConfirmPassword, new { @class = "form-control" })
        </div>
    </div>
    <div class="form-group">
        @Html.LabelFor(m => m.RoleName, new { @class = "col-md-2 control-label" })
        <div class="col-md-10">
            
        </div>
    </div>
    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input type="submit" class="btn btn-default" value="Register" />
        </div>
    </div>
}

I tried to find materials related to it but have on results. Please help me in this problem. I really appreciate