Hello @adguah,
Please have a look at this blog post here https://pkp.sfu.ca/2018/03/01/enriching-ojs-omp-with-javascript-no-coding-required/,
As well as this forum post: Use Case/Examples for Custom header plugin for OJS - #19 by Carlos_Alberto_Rivad - for some tips around troubleshooting this plugin.
-Roger