Source

File: wp-includes/rest-api/endpoints/class-wp-rest-terms-controller.php

public function prepare_item_for_database( $request ) {
		$prepared_term = new stdClass;

		$schema = $this->get_item_schema();
		if ( isset( $request['name'] ) && ! empty( $schema['properties']['name'] ) ) {
			$prepared_term->name = $request['name'];
		}

		if ( isset( $request['slug'] ) && ! empty( $schema['properties']['slug'] ) ) {
			$prepared_term->slug = $request['slug'];
		}

		if ( isset( $request['taxonomy'] ) && ! empty( $schema['properties']['taxonomy'] ) ) {
			$prepared_term->taxonomy = $request['taxonomy'];
		}

		if ( isset( $request['description'] ) && ! empty( $schema['properties']['description'] ) ) {
			$prepared_term->description = $request['description'];
		}

		if ( isset( $request['parent'] ) && ! empty( $schema['properties']['parent'] ) ) {
			$parent_term_id   = 0;
			$requested_parent = (int) $request['parent'];

			if ( $requested_parent ) {
				$parent_term = get_term( $requested_parent, $this->taxonomy );

				if ( $parent_term instanceof WP_Term ) {
					$parent_term_id = $parent_term->term_id;
				}
			}

			$prepared_term->parent = $parent_term_id;
		}

		/**
		 * Filters term data before inserting term via the REST API.
		 *
		 * The dynamic portion of the hook name, `$this->taxonomy`, refers to the taxonomy slug.
		 *
		 * @since 4.7.0
		 *
		 * @param object          $prepared_term Term object.
		 * @param WP_REST_Request $request       Request object.
		 */
		return apply_filters( "rest_pre_insert_{$this->taxonomy}", $prepared_term, $request );
	}