I looked it up and it was easier than I thought:

DECLARE LONG LogonUserA in win32api string,string,string,long,long,long@
cUser = "User"
cDomain = "" && Blank for PDC
cPassword = "Devcon"

IF LogonUserA(cUser,cDomain,cPassword,3,0,0) = 1
=MessageBox("Login Successful")
ELSE
=MessageBox("Login Failed")
QUIT
ENDIF