Your application client secret is stored encrypted so we cannot retrieve the unencrypted version to tell you the value if you forget it.
You can reset it, which will update the stored value and return the new value to you.
To do that click 'Apps' in the main menu, click on the application in question and then you can click the 'Reset' link in the 'Client Secret' section.
Your new secret will be displayed at the top of the page.
Video tutorial: https://youtu.be/lfXRJpM72L4