Broken link

I am helping a friend with his website. https://www.roastedstone.com At the top of the page is a link to an order form that does not work. I had the same problem with the Blog link, but was able to fix it by updating the code to point to the correct url (not sure why it changed). I was able to find the correct url for the blog page because it was listed for me in WordPress under “Posts.” I am having no such luck with the “Order Form” page. The website was written using php which I do not know. I do know other languages so I can gleen a few things, but I am not understanding how php derives URLs. I know where to enter the new url for the Order Form page, I need to know what the new url is. I have found the orderform.php file in the file structure but linking directly to it causes an error:

‘Fatal error: Call to undefined function get_header() in /hermes/bosweb/web228/b2287/whl.roastedstone/www/wp-content/themes/roasted-stone/orderform.php on line 1’

Thank you in advance for your help.