In such a situation the first task is to look in the documentation. There are actually two functions with that name. They are documented here:
- http://docwiki.embarcadero.com/Libraries/XE5/en/System.AnsiStrings.AnsiStartsStr
- http://docwiki.embarcadero.com/Libraries/en/System.StrUtils.AnsiStartsStr
The title of the documentation tells you the unit you need. Either
- System.AnsiStrings or
- System.StrUtils
So you need to add the appropriate unit to your uses clause. I cannot tell for sure which function you need.
If you have problems searching the documentation try the advice here: How can I search for Delphi documentation? Of course you should just be able to click in the unrecognised identifier and press F1.