I understand that I need to add the class "colorbox-inline" to the link and build the url like 119 this "?width=500&height=500&inline=true#id-of-content"... but how do I do that? Do I need to edit a .tpl file?
CSS style the inline-content you want to display with "display: none;"
for colorbox: in "configuration", "Media", "Colorbox" --> advanced settings, activate "Enable Colorbox inline"
add this as the html code for opening the colorbox webform link:
<a class="colorbox-inline" href="?width=310&height=460&inline=true#name-of-your-inline-content">
If this is not working try some additionally jQuery-Magic: