I forgot that you want to use Add Mode. You'll need to skip the part about making the text box read-only unless you use Update Mode. Actually, you'll probably want to use Update Mode though, because in Add Mode you would have to enter a value in the text box and save the record before you could use Mass Update. Otherwise, there would be "0" records to update.
To use your custom menu in Update Mode, you can use the following code in the "On Application Open" event.
var success as Int
success = @SelectTreeItem(@Application + "!Forms!Search/Update!Menu!Menu")
success = @SelectTreeItem("Search Menu!Search Commands!Retrieve New Results (F10)")
SetDefaultTab("Search/Update Menu")
AddToTranslationTable("Search/Update Menu", "Main Menu")
This code assumes that your custom menu is named simply "Menu". The last line just renames the form's tab to more accurately reflect what the form is.
Also, I thought of this after I posted the previous message, rather than making all of the colors of the text box the same as the layout background, it would be easier to use the Visibility function to hide it. That way, you would still see it in design mode.