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

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.

Edit the template, and place a variable in the input field for the password. Assign your data to this variable.

Hmmm this would mean turning it into a product due to template editting but thats ok...
I just want the string next to the p/w boxes not in it,

Could you post some demo code on how to assign variable please...

No it wouldn't. You don't have to do automatic template edits. Manually edit the template, and place a variable there. Assigning a value to a variable is easy - it is also fundamental. ;)