defaults write /Applications/Stickies.app/Contents/Info LSUIElement 1

# Stickies is the name of the application, replace with the custom application
# Change 1 to 0 to turn the feature off


************** OR **********************

# Find the application, usually in the applications folder
# Right click and 'open package contents'
# From there, in the contents folder, open the Info.plist file
# There At the end paste 

<key>LSUIElement</key>
<string>1</string>

before 
</dict>
</plist>

# Restart the application