Old site host to new site page

I am trying to redirect an external domain to a specific page on our website. I have the external domain pointing to the website and it forwards to it fine. Now I need to be able to set up a redirect so when the person types in https://mytest.io it goes to a specific page on our website /ABC/. For some reason, I cannot get this to pickup the mytest.io and redirect it to /ABC/ can you please advise how to do this?

Many thanks,
Mike

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