Rita,
Don't use a leader tab for that. Set a tab on the right margin, type a
normal tab that goes to that tab set, select the tab character and apply
underline. If you are using formfields, apply underline to the formfield
that precedes the tab. If not, the user will have to know that they need to
turn underline on before typing. Try this out to see how it works and then
let me know if that doesn't meet your needs.
Regards,
Chad DeMeyer