[How 2..?] Add php code to template with plugin

[Stagehandspace]

I've created a plugin (not product) which adds a random password string to the register page generated by php,

it shows ok but its stuck up in the top left corner of the page...how can I now add a "placement" hook so it will show in the password area of the register page?

I know about this:
How to Include a PHP or HTML File
but I want only to use the plugin system for this to work and not have to upload an external file to server or/and if possible not add a new template.