Code Modified

<script>
//STEP 1: Update CUSTOMFIELDMERGE with the actual customfield merge field eg. ~Contact._CustomFieldName~
//Step 2: Change VALUE1, VALUE2, etc... with the ones in the custom field
//Step 3: Set your redirect urls
switch ( "~Contact._InterestedIn~" ) {
    case 'Infusionsoft':
        redirect = "http://example.com/infusionsoft";
        break;

    case 'Ontraport':
        redirect = "http://example.com/ontraport";
        break;

    case 'Active Campaign':
        redirect = "http://example.com/activecampaign";
        break;
    case 'Mail Chimp':
        redirect = "http://example.com/activecampaign";
        break;

    //USE THIS AS A FAILSAFE OR IF NONE OF THE CONDITIONS ABOVE ARE TRUE
    default:
        redirect = "http://example.com/othercrmsoftware";
        break;
}
window.location = redirect;
</script>