Open the template card for the data grid table by clicking on the Row Template button in the property inspector.

Drag a button onto the template card and name it My Header Behaviour.

Assign the script behaviour of the My Header Behaviour button to the script of button "Default Header" of stack "revDataGridLibrary":

set the script of the selobj to the script of the behavior of button "Default Header" of stack "revDatagridLibrary"

This ensures that your behaviour has the same basic behaviour that a column header should have.

Now edit the script of the My Header Behaviour button. You want to customize the dgColumn getProp.

Select your data grid and edit it's script. Define the SomeFunctionThatReturnsToolTip function to return a tooltip based on the column name passed in (remember to press "Apply").

function SomeFunctionThatReturnsToolTip pColumn
   switch pColumn
      case "col 1"
         return "A tooltip for column 1"
         break
      case "col 2"
         return "A tooltip for column 2"
         break             
   end switch
end SomeFunctionThatReturnsToolTip

To finish up you just need to assign the "default header behavior" property to the long id of the My Header Behaviour button, which is on a different stack, which requires getting the name of the stack:

This can be done by going onto the template stack, then typing into the Message Box:

put the name of this stack

Then insert the name of the stack in <stack name>:

set the dgProps["default header behavior"] of selobj() to the long id of button "My Header behaviour" of stack "<stack name>"

This tells the data grid to use the script of your button rather than the default script supplied with the data grid.

Select the data grid (1), then select the 4th tab across (2), then finally add 2 columns using the + button (3).

While still in the property inspector, select the 2nd tab across (1), then select "Refresh Data Grid" (2).

Now when you roll over a column header the tooltip will appear.