The short answer is that you should use a hyphen for your SEO URLs. Google treats a hyphen as a word separator, but does not treat an underscore that way. Google treats and underscore as a word joiner — so red_sneakers is the same as redsneakers to Google. … Again, SEO URLs should use hyphens to separate words.
For More Information Please Refer: https://www.ecreativeim.com/blog/index.php/2011/03/30/seo-basics-hyphen-or-underscore-for-seo-urls/