How to replace sting value with variable.....

In the wonderful world of Exchange 2010 running the command below is successful evaluating and exporting email messages with the subject of "ozzzzzy". 

New-MailboxExportRequest -Mailbox $dlist.alias -Contentfilter {(subject -like "ozzzzy")} -FilePath $PSTpth }

So in my effort to build a GUI to this command, replacing "ozzzzzy" with a string variable from a textbox has proven in-effective. (Doesn't work) 

I have tried a multitude of different methods to manipulate this string / texbox value. I must be missing something very  fundamental about PowerShell that's not allowing my to substitute this parameter / value. 

I've tried this local to the Exchange box as well as remote. 

Your idea's are greatly appreciated. Thank you.....

Djoe