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.
First, find your voice in My Voices, and select 'Use Voice' to open Speech Synthesis with your voice selected.
Enter text of your choice between 70-149 characters and click 'Generate speech'.
Return to My Voices, find your voice in the list and select it to open up the detailed view.
Click on the 'Share' icon:
If you have already shared your voice, the icon will look different:
In the Voice Model Sharing options, click the toggle to enable sharing, then enable Discovery in Voice Library.
In the Voice Library Sharing options, 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 70-149 characters) will be available to select.