Hello. First of all, let me warn you up front that I don't have a lot of programming knowledge.

I followed the instructions on the LimeSurvey website to use the JavaScript feature, but I don't see the server button in my editor toolbar.
( manual.limesurvey.org/Workarounds:_Manip...tc..29_in_LimeSurvey ) 

Do you know if there is any problem or am I missing something?
I'm sending a screenshot for understanding 

Thanks for your help! 

Yes, you are missing something.
You missed to hover over the buttons to see the tooltips to get familiar with the editor.
 
 

Thanks for answering!
I entered the source button, and this is the tab that comes up. Do you know how to save the code that I've entered here? 

Thanks for answering!
I entered the source button, and this is the tab that comes up. Do you know how to save the code that I've entered here?

The usual way.
At the bottom of that window there should be two buttons "OK" and "Cancel"
You did not show the whole screen
And afterwards you save the entire question with the "Save" button.

But as you use the Popup HTML editor, you are in source mode initially, and by clicking the pencil you switch to WYSIWYG mode.

 

I'll give it a try, thank you so much!