WP_Widget::get_field_id( string $field_name )

Source

File: wp-includes/class-wp-widget.php

public function get_field_id( $field_name ) {
		$field_name = str_replace( array( '[]', '[', ']' ), array( '', '-', '' ), $field_name );
		$field_name = trim( $field_name, '-' );

		return 'widget-' . $this->id_base . '-' . $this->number . '-' . $field_name;
	}