How to connect an external js file in php file?

Good day!
The thing is, trying to change 404 page. Now the page content is displayed so prntscr.com/jacctm
in the field $message will Staley your code and it appears, BUT the new page contains the search, which works due to the script, and after inserting the script everything is broken and displays a blank page. Tried to escape the script code to the machine normally did, but nothing helps. Thought there may be a way to connect this script from the outside? or MB all you need is to bring in a php format and mount as a file .php?
Again, if stiti html code with css styles - working fine as just adding script things fall apart.
I will be glad tips and ideas)