diff --git a/classes/controllers/FrmFormsController.php b/classes/controllers/FrmFormsController.php
index 8eef149989..6af5c064b8 100644
--- a/classes/controllers/FrmFormsController.php
+++ b/classes/controllers/FrmFormsController.php
@@ -1066,6 +1066,27 @@ public static function build_new_form() {
 		 */
 		$new_values = apply_filters( 'frm_new_form_values', $new_values );
 
+		// Match the shape FrmFormsHelper::setup_new_vars() gives this filter, so a callback can
+		// safely read an existing key (e.g. before_html) before modifying it either way.
+		$new_values = FrmFormsHelper::fill_default_opts( $new_values, false, $new_values );
+
+		/**
+		 * Allows overriding a new form's default option values (before_html, submit_html, etc).
+		 *
+		 * @since 6.36
+		 *
+		 * @param array $values Form values.
+		 */
+		$new_values = apply_filters( 'frm_setup_new_form_vars', $new_values );
+
+		// FrmForm::create() reads default option values from $new_values['options'], not top level.
+		foreach ( FrmFormsHelper::get_default_opts() as $var => $default ) {
+			if ( isset( $new_values[ $var ] ) ) {
+				$new_values['options'][ $var ] = $new_values[ $var ];
+			}
+			unset( $var, $default );
+		}
+
 		$form_id = FrmForm::create( $new_values );
 		/**
 		 * @since 5.3
diff --git a/tests/phpunit/forms/test_FrmFormsControllerAjax.php b/tests/phpunit/forms/test_FrmFormsControllerAjax.php
index 31ba3c56a7..b9522c83ed 100644
--- a/tests/phpunit/forms/test_FrmFormsControllerAjax.php
+++ b/tests/phpunit/forms/test_FrmFormsControllerAjax.php
@@ -96,4 +96,69 @@ private function _check_updated_values( $form_id ) {
 			$this->assertSame( $posted_val, $actual_val, 'The default value was not updated correctly for field ' . $field->field_key . '.' );
 		}
 	}
+
+	/**
+	 * @covers FrmFormsController::build_new_form
+	 * with ajax
+	 */
+	public function test_build_new_form_applies_frm_setup_new_form_vars_filter() {
+		add_filter( 'frm_setup_new_form_vars', array( $this, '_set_custom_before_html' ) );
+
+		$_POST = array(
+			'action' => 'frm_install_form',
+			'nonce'  => wp_create_nonce( 'frm_ajax' ),
+			'name'   => 'Vivi Setup New Form Vars Test',
+			'desc'   => '',
+		);
+		$_REQUEST = $_POST;
+
+		$response = json_decode( $this->trigger_action( 'frm_install_form' ), true );
+
+		remove_filter( 'frm_setup_new_form_vars', array( $this, '_set_custom_before_html' ) );
+
+		$this->assertNotEmpty( $response['redirect'] ?? '', 'build_new_form did not return a redirect URL.' );
+		parse_str( (string) wp_parse_url( $response['redirect'], PHP_URL_QUERY ), $redirect_args );
+
+		$form = FrmForm::getOne( $redirect_args['id'] );
+		$this->assertNotEmpty( $form, 'Form not found with id ' . $redirect_args['id'] );
+		$this->assertSame( 'VIVI_TEST_MARKER', $form->options['before_html'], 'frm_setup_new_form_vars did not affect the created form.' );
+	}
+
+	public function _set_custom_before_html( $values ) {
+		$values['before_html'] = 'VIVI_TEST_MARKER';
+		return $values;
+	}
+
+	/**
+	 * @covers FrmFormsController::build_new_form
+	 * with ajax
+	 */
+	public function test_build_new_form_frm_setup_new_form_vars_callback_can_read_existing_key() {
+		add_filter( 'frm_setup_new_form_vars', array( $this, '_append_to_before_html' ) );
+
+		$_POST = array(
+			'action' => 'frm_install_form',
+			'nonce'  => wp_create_nonce( 'frm_ajax' ),
+			'name'   => 'Vivi Append Before Html Test',
+			'desc'   => '',
+		);
+		$_REQUEST = $_POST;
+
+		$response = json_decode( $this->trigger_action( 'frm_install_form' ), true );
+
+		remove_filter( 'frm_setup_new_form_vars', array( $this, '_append_to_before_html' ) );
+
+		$this->assertNotEmpty( $response['redirect'] ?? '', 'build_new_form did not return a redirect URL.' );
+		parse_str( (string) wp_parse_url( $response['redirect'], PHP_URL_QUERY ), $redirect_args );
+
+		$form = FrmForm::getOne( $redirect_args['id'] );
+		$this->assertNotEmpty( $form, 'Form not found with id ' . $redirect_args['id'] );
+		$expected = FrmFormsHelper::get_default_html( 'before' ) . '_APPENDED';
+		$this->assertSame( $expected, $form->options['before_html'], 'Callback could not read the existing before_html default.' );
+	}
+
+	public function _append_to_before_html( $values ) {
+		$values['before_html'] .= '_APPENDED';
+		return $values;
+	}
 }