Check a Form with a Textfield
I'd like to check a Textfield if the value is already in a database list.
If it is, it should not be possible to submit the form.
Best: When I enter a name that exist there is somewhere the feedback: This name already exists. Choose an other name.
I know this functionality from other vendors - there you can add PowerShell Scripts to validate the input.
Maybe anyone can point me into the rigth direction - maybe with some code.