simple shopify plugin to embed our chat widget js into footer or header so it shows on peoples pages

need a simple shopify plugin to embed our chat js into footer or header so it shows on peoples pages
need to add a variable to the code that relates to the web address its like this:
if the domain is
http://www.domain.com.au
it becomes
domaincomau
here are some examples:
ww2.test.co.nz
testconz
http://happy.org/shop
happyorg