When you build the URL string in your HTML mail.. Why not just have a standard set of text, predefined in your routine.. Where the user name portion of the text would be a string like REPLACE_ME.. When you need to send the e-mail, just use a replace command in pl/sql to find the REPLACE_ME string with your username vale..
you can use a Page Item as substitution variable within the Mail HTML Body.
e.g. bla bla bla &P123_MY_PAGE_ITEM. bla bla bla
The Syntax is an ampersand in front of the page item name and a dot after the name.
thanks its working fine