Javascript – How to add spinner to ASP.net while processing

I am using ASP.net Web forms

I have users register into my page

once they registered a 3rd pty code updates the database which can take between 1 sec and few minutes

I want to add a rotating spinner wheel while server is running and waiting for the 3rd pty response.

there is a loop with waiting to check the for 3rd pty result

what i want, is to have a spinner that shows the user that there is a process running in the backend

I tried to add asp:image with spinning gif, but when i call it it does not show till the process is finished

i wonder if there is an easy way to add the spinner

here is my code for reg.aspx

<div>
<div>Register</div>
<div>
    <asp:TextBox ID="FirstName" runat="server" placeholder="First Name"></asp:TextBox>
</div>
<div>
    <asp:TextBox ID="LastName" runat="server" placeholder="Last Name"></asp:TextBox>
</div>
<div>
    <asp:TextBox ID="PhoneNumber" runat="server" placeholder="Phone Number"></asp:TextBox>
</div>
<div>
    <asp:TextBox ID="Email" runat="server" placeholder="Email" TextMode="Email"></asp:TextBox>
</div>
<div>
    <asp:TextBox ID="Password" runat="server" placeholder="Password" TextMode="Password"></asp:TextBox>
</div>
    <div>
    <asp:Label ID="ErrorMessage" runat="server" Visible="false" Text=""></asp:Label>
</div>
<div>
    <asp:Image runat="server" ID="imgSpin" ImageUrl="~/images/spinner6.gif" Visible="true" />
</div>                    

<asp:Button ID="registerButton" runat="server" Text="Sign Up" OnClick="registerButton_Click" />

</form>
</body>

and here for reg.aspx.cs

protected void registerButton_Click(object sender, EventArgs e)
{
    RegisterUser();
}

   protected void RegisterUser()
   {
       imgSpin.Visible = true;
       
       string firstName = FirstName.Text;
       string lastName = LastName.Text;
       string phoneNumber = PhoneNumber.Text;
       string email = Email.Text;
       string password = Password.Text;

       Reg(firstName, lastName, phoneNumber, email, password);
       string resultCode = "*";
       bool bExit = false;


       while (!bExit)
       {
           switch (resultCode.ToUpper())
           {
               case "ERROR":
                   ErrorMessage.Text = "Invalid user ID or password. Please try again.";
                   ErrorMessage.Visible = true;
                   bExit = true;
                   break;


               case "OK":
                   Response.Redirect("job.aspx");
                   bExit = true;
                   break;

               default:
                   // Wait 2 seconds, then call the procedure again
                   System.Threading.Thread.Sleep(2000);
                   resultCode = Call3Pty(); 
                   break;
           }
       }

       imgSpin.Visible = false;
   }

GPT suggested adding 2 event to the Signup button but that never works, only client side gets triggered but the backend never did

<asp:Button ID="registerButton" runat="server" Text="Sign Up" OnClick="registerButton_Click" OnClientClick="showSpinner();" />

how can i add a simple spinner while my code running in the background