RE: Javascript to Hive KeyChain - how to ask to grant posting authority?

Drop the form tags. You don't need them. The form tags are for when you would load a new page from a script into the browser replacing the HTML and the Javascript you are depending on! You don't want to do that.

You should use requestAddAccountAuthority and requestTransfer. Check: https://github.com/stoodkev/hive-keychain

I'll just plug my website here while I am at it. Manage your permissions here: https://www.steemfiles.com/@leprechaun/permissions