No. The custom value option doesn't allow you to type in whatever value you want. Instead, custom values allow you to send a different value from your trigger app each time a Zap runs.
For example, if you're selecting a label for your Trello card and you don't see the color you're looking for in the dropdown menu, you cannot select Use a Custom Value and type in the new color.
Learn how to add custom values to dropdown menu fields in Zaps.