User registration event ASP.NET Core 3 - asp.net-core

Question

I've created an ASP.NET Core 3 website with all the latest updates (Visual Studio 2019). I would like to perform certain actions when a user registers but I cannot find out how to do that. I suspect that I have to hook up to an event in my start up class, but all my googling has left me in loops. Please assist. My current startup class looks like this (it's mostly the generated code)
public void ConfigureServices(IServiceCollection services)
{
services.TryAddTransient<ITokenService, CustomTokenService>();
services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
services.AddDefaultIdentity<ApplicationUser>().AddEntityFrameworkStores<ApplicationDbContext>();
services.AddIdentityServer().AddApiAuthorization<ApplicationUser, ApplicationDbContext>();
services.AddAuthentication().AddIdentityServerJwt();
services.AddControllersWithViews();
services.AddRazorPages();
services.AddSpaStaticFiles(configuration =>
{
configuration.RootPath = "ClientApp/dist";
});
}
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
app.UseDatabaseErrorPage();
}
else
{
app.UseExceptionHandler("/Error");
app.UseHsts();
}
app.UseAuthentication();
app.UseIdentityServer();
app.UseAuthorization();
app.UseHttpsRedirection();
app.UseStaticFiles();
if (!env.IsDevelopment())
{
app.UseSpaStaticFiles();
}
app.UseRouting();
app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(name: "default", pattern: "{controller}/{action=Index}/{id?}");
endpoints.MapRazorPages();
});
app.UseSpa(spa =>
{
spa.Options.SourcePath = "ClientApp";
if (env.IsDevelopment())
{
spa.UseAngularCliServer(npmScript: "start");
}
});
}