diff --git a/modules/ppcp-settings/resources/js/Components/Screens/Onboarding/Components/ManualConnectionForm.js b/modules/ppcp-settings/resources/js/Components/Screens/Onboarding/Components/ManualConnectionForm.js
index 9524ed553..6602c335b 100644
--- a/modules/ppcp-settings/resources/js/Components/Screens/Onboarding/Components/ManualConnectionForm.js
+++ b/modules/ppcp-settings/resources/js/Components/Screens/Onboarding/Components/ManualConnectionForm.js
@@ -96,7 +96,7 @@ const ManualConnectionForm = () => {
 	// On-the-fly form validation.
 	useEffect( () => {
 		setClientValid(
-			! manualClientId || /^[\w-]{80,}$/.test( manualClientId )
+			! manualClientId || /^A[\w-]{79}$/.test( manualClientId )
 		);
 		setSecretValid( manualClientSecret && manualClientSecret.length > 0 );
 	}, [ manualClientId, manualClientSecret ] );
diff --git a/modules/ppcp-settings/src/Endpoint/AuthenticationRestEndpoint.php b/modules/ppcp-settings/src/Endpoint/AuthenticationRestEndpoint.php
index 23f08631b..55a3b8353 100644
--- a/modules/ppcp-settings/src/Endpoint/AuthenticationRestEndpoint.php
+++ b/modules/ppcp-settings/src/Endpoint/AuthenticationRestEndpoint.php
@@ -88,6 +88,7 @@ class AuthenticationRestEndpoint extends RestEndpoint {
 						'type'              => 'string',
 						'sanitize_callback' => 'sanitize_text_field',
 						'minLength'         => 80,
+						'maxLength'         => 80,
 					),
 					'clientSecret' => array(
 						'required'          => true,
diff --git a/modules/ppcp-settings/src/Service/AuthenticationManager.php b/modules/ppcp-settings/src/Service/AuthenticationManager.php
index fc3d8da74..95acf068b 100644
--- a/modules/ppcp-settings/src/Service/AuthenticationManager.php
+++ b/modules/ppcp-settings/src/Service/AuthenticationManager.php
@@ -147,7 +147,7 @@ class AuthenticationManager {
 			throw new RuntimeException( 'No client ID provided.' );
 		}
 
-		if ( false === preg_match( '/^[\w-]{80,}$/', $client_secret ) ) {
+		if ( false === preg_match( '/^A[\w-]{79}$/', $client_secret ) ) {
 			throw new RuntimeException( 'Invalid client ID provided.' );
 		}