Log into the Azure Portal here then go to 'App registration' and select your newly registered application.
On the left-hand side, select 'Certificates & secrets' and then 'New client secret'.
Give the secret a 'Description' such as 'Reading Cloud' then set the 'Expired' for the time frame you would like the secret to last.
Click 'Add' and the secret will now display.
Under 'Value' select the copy icon, then in another window in your browser, ensure you have selected your identity provider. Paste the secret into the 'Client Secret' box and press 'OK' at the bottom of the box to save your settings.