diff --git a/src/class-convertkit-api-v4.php b/src/class-convertkit-api-v4.php
index 4fbd9c1..3b14c52 100644
--- a/src/class-convertkit-api-v4.php
+++ b/src/class-convertkit-api-v4.php
@@ -248,13 +248,7 @@ private function generate_and_store_code_verifier() {
 		$code_verifier = random_bytes( 64 );
 
 		// Encode to Base64 string.
-		$code_verifier = base64_encode( $code_verifier ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions
-
-		// Convert Base64 to Base64URL by replacing “+” with “-” and “/” with “_”.
-		$code_verifier = strtr( $code_verifier, '+/', '-_' );
-
-		// Remove padding character from the end of line.
-		$code_verifier = rtrim( $code_verifier, '=' );
+		$code_verifier = $this->base64_urlencode( $code_verifier );
 
 		// Store in database for later use.
 		update_option( 'ck_code_verifier', $code_verifier );
@@ -317,15 +311,38 @@ private function delete_code_verifier() {
 
 	}
 
+	/**
+	 * Base64URL encode the given string.
+	 *
+	 * @since   2.0.0
+	 *
+	 * @param   string $str    String to encode.
+	 * @return  string         Encoded string.
+	 */
+	public function base64_urlencode( $str ) {
+
+		// Encode to Base64 string.
+		$str = base64_encode( $str ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions
+
+		// Convert Base64 to Base64URL by replacing “+” with “-” and “/” with “_”.
+		$str = strtr( $str, '+/', '-_' );
+
+		// Remove padding character from the end of line.
+		$str = rtrim( $str, '=' );
+
+		return $str;
+
+	}
+
 	/**
 	 * Returns the URL used to begin the OAuth process
 	 *
 	 * @since   2.0.0
 	 *
-	 * @param   bool|string $state  Optional state parameter to include in OAuth request.
-	 * @return  string                  OAuth URL
+	 * @param   bool|string $return_url  Return URL.
+	 * @return  string                       OAuth URL
 	 */
-	public function get_oauth_url( $state = false ) {
+	public function get_oauth_url( $return_url = false ) {
 
 		// Generate and store code verifier and challenge.
 		$code_verifier  = $this->generate_and_store_code_verifier();
@@ -340,9 +357,15 @@ public function get_oauth_url( $state = false ) {
 			'code_challenge_method' => 'S256',
 		);
 
-		// If a state parameter needs to be included, add it now.
-		if ( $state ) {
-			$args['state'] = rawurlencode( $state );
+		if ( $return_url ) {
+			$args['state'] = $this->base64_urlencode(
+				wp_json_encode(
+					array(
+						'return_to' => $return_url,
+						'client_id' => $this->client_id,
+					)
+				)
+			);
 		}
 
 		// Return OAuth URL.
diff --git a/tests/wpunit/APITest.php b/tests/wpunit/APITest.php
index 4316f86..f14af87 100644
--- a/tests/wpunit/APITest.php
+++ b/tests/wpunit/APITest.php
@@ -351,7 +351,7 @@ public function testGetOAuthURLWithState()
 	{
 		// Confirm the OAuth URL returned is correct.
 		$this->assertEquals(
-			$this->api->get_oauth_url( 'an-example-state' ),
+			$this->api->get_oauth_url( 'https://example.com' ),
 			'https://app.convertkit.com/oauth/authorize?' . http_build_query(
 				[
 					'client_id'             => $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
@@ -359,7 +359,14 @@ public function testGetOAuthURLWithState()
 					'redirect_uri'          => $_ENV['CONVERTKIT_OAUTH_REDIRECT_URI'],
 					'code_challenge'        => $this->api->generate_code_challenge( $this->api->get_code_verifier() ),
 					'code_challenge_method' => 'S256',
-					'state'                 => 'an-example-state',
+					'state'                 => $this->api->base64_urlencode(
+						wp_json_encode(
+							array(
+								'return_to' => 'https://example.com',
+								'client_id' => $_ENV['CONVERTKIT_OAUTH_CLIENT_ID'],
+							)
+						)
+					),
 				]
 			)
 		);