I don't understand why you need the FindWindow call. Try this simplified version. It works fine on my machine.
If it works, you need to change the hardcoded URL.

tcUrl = IIF(type("tcUrl")="C",tcUrl,"https://www.vfphelp.com/vfp9/index.htm")
tcAction = IIF(type("tcAction")="C",tcAction,"Open")
IF type("Application.StartMode") = "N" AND !inlist(Application.StartMode,2,3,5)
  *  UI only when not being called from a COM server
  WAIT WINDOW "Loading " + tcURL + " ..." NOWAIT
ENDIF
DECLARE INTEGER ShellExecute IN SHELL32.dll ;
     INTEGER nWinHandle, ;
     STRING cOperation, ;
     STRING cFileName, ;
     STRING cParameters, ;
     STRING cDirectory, ;
     INTEGER nShowWindow
RETURN ShellExecute(0,tcAction,tcUrl,"",'',1)

>Hi Tore
>
>this line
>
>RETURN ShellExecute(FindWindow(0,_Screen.Caption),tcAction,tcUrl,"",SYS(2023),1)
>
>>>Hello all,
>>>
>>>I will have a button on form. When the user clicks that button I want to launch where the user can make it's payment.
>>>
>>>My app is on WIndows Server 2016.
>>>
>>>So far someone gave me this code
>>>
>>>

>>>lParameters tcURL, tcAction
>>>tcUrl = IIF(type("tcUrl")="C",tcUrl,"https://square.link/u/HereCodeForThePersonWhowillBePaid")
>>>tcAction = IIF(type("tcAction")="C",tcAction,"Open")
>>>IF type("Application.StartMode") = "N" AND !inlist(Application.StartMode,2,3,5)
>>>  *  UI only when not being called from a COM server
>>>  WAIT WINDOW "Loading " + tcURL + " ..." NOWAIT
>>>ENDIF
>>>DECLARE INTEGER ShellExecute IN SHELL32.dll ;
>>>     INTEGER nWinHandle, ;
>>>     STRING cOperation, ;
>>>     STRING cFileName, ;
>>>     STRING cParameters, ;
>>>     STRING cDirectory, ;
>>>     INTEGER nShowWindow
>>>DECLARE INTEGER FindWindow IN WIN32API ;
>>>     STRING cNull, ;
>>>     STRING cWinName
>>>RETURN ShellExecute(FindWindow(0,_Screen.Caption),tcAction,tcUrl,"",SYS(2023),1)
>>>

>>>
>>>First time I executed that code I received this : Declare DLL caused an exception
>>>Second time I executed my VFP session froze.
>>>
>>>Suggestion?
>>>
>>>Regards
>>
>>Which line gave the error?