We need to implement a web service request via AMPSCRIPT/SSJS to an external system.
These requests have to be signed by a certificate in order to be trusted by external system.
Can Marketing Cloud use signed http request?
You need to raise a request with support to have this added. It does incur a charge, and the SSL certificate will be added to a custom domain.
Excerpt from documentation
If you're interested in having SSL configured for your Marketing Cloud account, please reach out to your Account Executive for pricing. Once the correct product(s) have been sold, you'll be engaged via a case requesting the following information:
Complete and return the SSL Request Form. Which of the above two functions will the SSL be used for? Will you require calls for HTTP to be force redirected to HTTPS? Will you be purchasing the Certificate or would you like us to purchase it at no additional charge?
You can find more information on it HERE