These forums are for the Bing Maps developer API's and not related to the Bing Maps consumer website. I recommend posting in the consumer forums here:
https://answers.microsoft.com/en-us/bing/forum/bing_maps
From what I can see, the embedded maps on the Bing Maps website don't provide routing functionality, only a single pushpin map for use on a contact us type page.
The Custom Map URL is only for directing users to the Bing Maps consumer site with pre-filled queries. The consumer site does not allow loading the site via an iframe.
If you want to show directions on your website, use the Bing Maps developer APIs.
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