Do you mean that if you select "box" from the drop down list, you want to find a record where the text in the field is "red box"? and also "red Boxes"?
assuming that your field with the drop down has global storage specified and the following script is performed after selecting text from your drop down list:
Enter Find Mode 
Set Field [YourTable::textField ; "*" & YourTable::globalDropDownListField & "*" ]
Set Error capture [on]
Perform Find 
For more examples of this kind of scripted find: Scripted Find Examples
Then use the script that I provided as an example of what you need to set up.
But still I am unable to get the search what I want. After applying your above script feed. where I am wrong pls refer to below link.http://dl.dropboxusercontent.com/u/15453414/Invoices.fmp12