Hi Eric-
The validation rules in the form Page Builder only allows the starts with…not “does not start with” option.
I am not sure if a work around…I also don’t know how your app is created for this functionality.
If you are unfamiliar with for validation rules you can go into your form in the Page Builder, selecting “Rules” then selecting the text filed and then “starts with”. This will default to “custom” and then a form filed you can type in your word.
Add a message when it doesn’t start with select