Place this snippet in an HTML field in your form:
<script> jQuery( document ).ready(function() { jQuery('input:radio[name="input_2"]').change( function(){ if (jQuery(this).is(':checked') && jQuery(this).val() == 'Enter Amount') { jQuery('#input_2_1').focus(); } }); }); </script>
Customize the above code according to your specific fields.
To add a regular text link that focuses on a specific field use the following code (add as a link and customize):
href="javascript:document.getElementById('input_2_1').focus()"