To lock and unlock Tell Me When, type "/tmw" or "/tellmewhen".
When you first log in with Tell Me When installed, you will see one group of four icons in the upper left hand corner of your screen.
This allows you to combine many different icon into one, central spot instead of many different places.
Personally, I have a whole group of icons set up for special situations that all feed into one meta icon that is very large and directly under my target's unit frame.
If the recipient is using version 4.0.0 of Tell Me When or later, they will be notified of your request and will be able to import your icon from the dropdown.
Icons can also be spatially manipulated by holding down the right mouse button and dragging them around.
Multiple units can be separated with semicolons, and more information about units can be found here and here.