>I just tried do (_beautify) in a window (with ON KEY LABEL), but I got an error message. Apparently, some parameter(s) has to be added after do (_beautify), but I have no idea which ones.

I believed the code for the beautifier should be in the xSource directory, but it's not there, at least not in that form. It could be a part of the FoxDoc (aka documenting wizard, but still named FoxDoc in xSource), but I won't dig in there - I like it as it is. Anyway, here's what the help says:

You can also specify a beautification application in your configuration file by including a line using this syntax:

_BEAUTIFY = cProgramName
Note that your beautification application must accept a single parameter, the name of the program to beautify. Your beautification application can modify the program, write the modifications to a temporary file, and then return the name of the temporary file to FoxPro. FoxPro reads the temporary file back into the editing session under the original FoxPro program name and then deletes the temporary file.

Though, I've tried to

DO (_beautify) WITH FULLPATH('a.prg')

but it errored out in beautify.init, with 'Function argument value, type, or count is invalid.'