When clicking on the Fetion computer client to send a text message, it displays "Let me guess who you want to send it to."

I just personally tested Halo: This is the prompt that appears when you enter the text message editing page using the PC function

You clicked Send without adding a contact, so the system prompts

At the top of the picture you posted (message editing dialog box) there should be a list of recipients to the right of the recipient address bar

Enter the recipient number manually or Just select it in the list and click Send