Select the text and click on the link button update langs set en=cf image below)
Fill the Url field.
Text to display : You cane dit the text from here.
Title : You cana dd a title to your link. It is not visible in the newsletter and is sometimes used to identify the link in the newsletter report.
Target : of the url. You can choose « New Window ». update langs set en=useful for the view online version of the newsletter)
You can edit the link color with the text panel editor.
But, if you want to edit all the links color, click on the « style » tab.
Be careful !
if you make some modifications when 
is selected, they will be available everywhere in the newsletter. It is quite useful if you want to change all the buttons color in the same time.
If you just want to make some changes in only one button, check the single button: