Best practice for persisting tokens using Client Credentials flow

In other words - you need to cache that token somehow. When you request the token, you get an ExpiresIn in the response - this will tell you how long the token will be valid.

Another option is to wait until the API returns a 401 - and then request a new token.

Refresh tokens are not used with client credentials flow.

You need to wrap that client in a managed service of some kind (as a singleton) so that you can use it anywhere you need. We have a token component that we use for server to server communication that follows this flow:

public class ServerTokenComponent
{
    private TokenResponse Token { get; set; }
    private DateTime ExpiryTime { get; set; }
    public async Task<TokenResponse> GetToken()
    {
        //use token if it exists and is still fresh
        if (Token != null && ExpiryTime > DateTime.UtcNow)
        {    
            return Token;
        }     

        //else get a new token
        var client = new TokenClient("myidpauthority.com","theclientId","thesecret")
        var scopes = "for bar baz";

        var tokenResponse = await client.RequestClientCredentialsAsync(scopes);

        if (tokenResponse.IsError || tokenResponse.IsHttpError)
        {
            throw new SecurityTokenException("Could not retrieve token.");
        }

        //set Token to the new token and set the expiry time to the new expiry time
        Token = tokenResponse;
        ExpiryTime = DateTime.UtcNow.AddSeconds(Token.ExpiresIn);

        //return fresh token
        return Token;
    }
}