how to link html to js? mine is not working

I've answered your question in this topic: https://feedback.textasticapp.com/communities/1/topics/1333-linking-html-and-css-files

The important part is to make sure that your HTML and JS file is stored in the same folder somewhere in "Local Files", "iCloud" or in a folder added using "Add External Folder…". 

Also, make sure to use regular quotation marks for the src attribute:

<script src="script.js"><script>