You can achieve this by using the -mask parameter:
In your case this would be:
New-UDTextbox -Mask "00000000"
The problem? Apparently adam wants to get rid of this parameter which doesnt work with regex as stated in the docs… it only works well with pattern mask…
In addition I realized that if you use -mask the -onvalidate won’t work in conjunction anymore. So unfortunately you can’t have both things together (at least at the moment).
I really hope that -mask will not disapper because based on some use cases I like this one better…