Are you editing "Application Manifest" file. Please let me know the exact steps, that you are performing, maybe a screenshot helps.
Generally these Manifest files contain xml content or key-value pairs. What actions you would like to perform in Manifest file? If you are trying to edit xml content of Manifest file, you can use XmlDocumentComponent from toolbar. If Manifest contains key-value pairs and If you need to edit the content, you may use windows adapter to open Manifest in notepad to exit. In this case,
Path= should be notepad.exe file path(eg: C:\Windows\System32\notepad.exe)
Arguments= path of the Manifest file(eg: C:\Application\Manifest.txt)
Interrogate the textbox(content) and use stringUtils to modify the content.