GetPartyLeaderIndex() will return 0 if the leader is not in your party, so unfortunately that will not work. IsRaidLeader() only checks yourself as the leader, what you need to use is

GetRaidRosterInfo(index)

the second agrument (rank) will return 2 if they are the leader. The index is a number 1-40.

Here's how to find the leaders name and whisper them using the default language for your faction.

Code:
/run for i=1,40 do local n,r,_,_,_,_,_,_,_,_,_ = GetRaidRosterInfo(i);if (r==2)then SendChatMessage("Please Promote Me", "WHISPER", nil, n);