Is it correct to request access token upon EVERY API call in my web app controllers?

I have a web API on the backend, and a web app and phone app on the front end.
I am using Auth0 for OAuth/OpenId authentication on all three.
Both web API and web app are written in ASP .Net Core 1.1 MVC.

Every time I call the web API from the web app, I request an access token from Auth0. So, in every controller action that requires access to my API, I have:

var client = new HttpClient();
var tokenResponse = await client.PostAsync(tokenEndpoint,
new FormUrlEncodedContent(new List<KeyValuePair<string, string>>
{
new KeyValuePair<string, string>("grant_type", "client_credentials"),
new KeyValuePair<string, string>("client_id", _auth0Settings.ClientId),
new KeyValuePair<string, string>("client_secret", _auth0Settings.ClientSecret),
new KeyValuePair<string, string>("audience", _auth0Settings.ApiIdentifier),
}));

Is this the correct way of doing this? Or should I just perform this once when the web app fires up, and then store the access token in a global variable that I can call from all my controllers?