you can add custom script easily from admin side
*Quick to set up
*Simple to insert scripts
*Insert header code and/or footer code
*Add Google Analytics code to any theme
*Add custom CSS across themes
*Add any Website tracking code
*Add Advertisement script code
*Insert Facebook pixel code
Extract the zip file and just drop the contents in the wp-content/plugins/ directory of your WordPress installation and then activate the Plugin from Plugins page.
After active plugin go to Setting->Insert Script In Headers And Footers.