You can generate a custom preview for your Professional Voice Clone, and choose this to be the preview for your voice in the Voice Library. You can change the custom preview at any time from the sharing settings for your voice.
First, you will need to have generated some suitable audio from a text prompt between 70 and 150 characters long.
If you don't already have any eligible generations, find your voice in My Voices, and use the T button to open Text to Speech with your voice selected. Enter text of your choice between 70-150 characters and click Generate speech.
When you have a generation you're happy with, return to My Voices. Find your voice in the list, click More actions (three dots) then select Share voice to access the sharing settings.
In the Publish to the Voice Library settings, use the drop down selection for Custom voice preview to select your recently generated text. This will then be applied as the preview for your voice. Only audio that meets the criteria (length of 70-150 characters) will appear in the list.