This domain is not a typical website you'd browse or buy from. It's an API gateway, likely part of a larger infrastructure on Amazon Web Services. The HTTPS certificate is valid and the connection is secure, but that's where the good news ends.
There's no public information about the company or people behind this service — no about page, no contact details, and the domain's WHOIS data is hidden because it's a subdomain of mtmeprod.com. That parent domain could be legitimate, but without being able to visit it or find documentation, you're flying blind.
If you're using this API as part of another service, make sure you trust who provided the endpoint. Check if the parent domain mtmeprod.com has a clear privacy policy and terms of use. For now, external-hornet.tmg-gateway.arms.use1.mtmeprod.com scores low on trust because of its opacity — you simply don't know enough to feel confident sending data its way.