How to get a different link dynamically for each option tag value?

How to get a different link dynamically for each option tag value?

I am creating a form through which customers can purchase an online service.
The form contains a dropdown list with a 'select' & 'option' tag.
The dropdown list is created to choose a billing cycle (6 months, 1 year,
2 years, 3 years)
I want to send the user to different addresses depending on the selection
of a billing cycle.
Eg. If 6 months is selected and "Order now" is clicked, the customer will
be send to link 1.
If 1 year is selected and "Order now" is clicked, the customer will be
send to link 2.
If 2 years is selected and "Order now" is clicked, the customer will be
sent to link 3.
I want to change the value of the href attribute dynamically for the ORDER
NOW button for each billing cycle.
How to do the same?