1. A simple addon like this might be a good chance to get your feet wet in addon development (especially as it's a very specific addon)
2. A simple functionality like this is a prime candidate for a macro you can drag to a button... you can even customize your whisper like you request in advanced features.
Addon is overkill in that sense.
/script msg="Please promote me to assistant and I'll use marks. Thanks" if not (IsRaidLeader() or IsRaidOfficer()) then SendChatMessage(msg,"WHISPER",nil,UnitName("target")) end;
This macro for example will /w your current target with msg.
A version that uses GetRaidRosterInfo to find the RL so that you don't have to target is also possible.
First macro is a 2-click to do what you want macro (click the RL unit frame to target him + click the macro)
Second would be 1-click.