When on a contact record in Hubspot you will see an option to "Start Live Chat" on the right side of the contact under the YetiText option. If the contact already has a live conversation you will see the conversation here and it will show "reply" instead of start live chat.
Once that option is clicked it will have you use your YetiText login and you can send a live text directly to that contact.
Here is what you will see if the contact has a conversation with your YetiText number already. You will click on 'Actions' and then reply to send a new message.