Deciding when to use a text box versus a personalized message, or both, usually depends on the subject matter within the text.
When to use an invitation personal message: If the information applies to the viewer. For example, ‘take a look at this before our meeting on Thursday’ would belong within the invitation. Your contact needs to know that before ever opening the ving.
When to use a text box component: If the information applies to the ving specifically. For example, ‘make sure you watch the video and answer the follow-up questions’ would belong in a text box. These are instructions pertaining to the content itself after they've already opened the ving.