Thanks for reaching out to Okadabooks.
To insert a clickable link in your book is very easy.
As you know, there are two options to publish on OkadaBooks: Creating an ePub or Uploading an ePub
1. If you're creating an ePub then go ahead and paste the link. Don't "paste as plain text" - Just paste and publish. The link will be clickable when you publish.
2. If you're uploading an epub instead then make sure the link in the epub is hyperlinked. The gif below will show you how to hyperlink an ePub
You can upload and publish or republish your book once done.
Click here to learn how to edit and republish your book