The problem is that single quotes are get lost somewhere during all this process. You can specify using "" which will be escaped to " in the end, e.g.:
ExternalMetric=RegQuery.EBTransfer720:""D:\Program
Files\NetXMS\custom\CheckRegKey\CheckRegKey.exe""
"HKLM\SOFTWARE\WOW6432Node\Aucotec\Engineering Base\7.2.0\Server\Files"
"ImageFilePath"
But better way is to use new (and not yet documented) docker-style syntax:
ExternalMetric=RegQuery.EBTransfer720:['D:\Program
Files\NetXMS\custom\CheckRegKey\CheckRegKey.exe',
'HKLM\SOFTWARE\WOW6432Node\Aucotec\Engineering Base\7.2.0\Server\Files',
'ImageFilePath']
currently that works with single quotes (') only, but in next patch release double quotes (") will also be supported.