If you want to create a product attribute programmatically, but not by admin then this blog post is right the solution for you.

Create file app/code/Magemonkey/CustomAttribute/Setup/InstallData.php

<?php
namespace MagemonkeyCustomAttributeSetup;

use MagentoEavSetupEavSetup;
use MagentoEavSetupEavSetupFactory;
use MagentoFrameworkSetupInstallDataInterface;
use MagentoFrameworkSetupModuleContextInterface;
use MagentoFrameworkSetupModuleDataSetupInterface;

class InstallData implements InstallDataInterface
{
	private $eavSetupFactory;

	public function __construct(EavSetupFactory $eavSetupFactory)
	{
		$this->eavSetupFactory = $eavSetupFactory;
	}
	public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
	{
		$eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);
		$eavSetup->addAttribute(
			MagentoCatalogModelProduct::ENTITY,
			'custom_attribute',
			[
				'type' => 'text',
				'backend' => '',
				'frontend' => '',
				'label' => 'Custom Atrribute',
				'input' => 'text',
				'class' => '',
				'source' => '',
				'visible' => true,
				'required' => true,
				'global' => MagentoEavModelEntityAttributeScopedAttributeInterface::SCOPE_GLOBAL,
				'user_defined' => false,
				'default' => '',
				'visible_on_front' => false,
				'used_in_product_listing' => true,
				'unique' => false,
				'apply_to' => '',
				'searchable' => false,
				'filterable' => false,
				'comparable' => false
			]
		);
	}
	
}