How to concatenate texts with Calculation?

I'm using a formula to concatenate alphanumeric strings entered via form with text used inside the calculation (eg. if I ask a website and want the form to automatically add "www" then the calculation is www + field1 where field1 is where the user enters the website. however, on saving, the www shows as switched off automatically on every edit. How do I get it to stay on? 

in my example, I'm making a utm generator that adds utm parameters to a url with url and parameters entered by user and standard utm identifier (eg. ?utm_campaign=) added in calculation.

You need to do 2 things to get this to work:

1. Since you're concatenating texts, you should omit the operators in between the fields.

2. Then you have to click on the actual field that has a black background so it's treated as a text.

Here's a short video showing those, along with the result: