563497 Copy and Replace Javascript Needed

Hello, I need a simple javascript that will copy the contents of one field to another and replace the spaces with underscores. For example, the field that will be filled in manually is called "businessname", and as we manually fill it, the field called "businesslink" should get auto filled but with underscores in place of the blank spaces. So if I enter "I Love You" on the first field, the "link" field should instantly display "I_Love_You". This should be done with a simple Javascript. A good programmer should be able to write this up in 5 minutes. I don't expect to pay more than $15 for this little snippet. Thanks, Jaime