I do not know how to do this using a web source script.
But you can try to modify your web source script to output the literal text ... 'Technical Credits:'$char(13)$char(10)
... just before outputting the credit info to be extracted from the web source.
See also: [WS] Discogs (pone mod)
Another way would be - when websource work is done - to use Mp3tag for what it is made for - use an action "Format value".
See Forum FAQ how to prepend resp. append some text to an existing tag-field content. Actions
You can change the content of the tag-field ARTIST using the Mp3tag Tag-Panel for all selected files at once.
Just type the new text into the edit field and press [Ctrl]+[S] to save resp. click the 'Save' icon.
Another way is to use Mp3tag for what it is made for - use an action "Format value".
To make sure to work only on files, which have a tag-field "CREDITS", you can set the Mp3tag Filter to get only those files displayed, where the tag-field CREDITS exist. NOT "%CREDITS%" IS ""
Not being well versed modifying scripts, I had actually tried your AD1 (Format Value) and the web source, before I got your response. The problem, I'm having is all the tag data is deleted and "Technical Credits" is add...
Using the correct terminology, I'm trying to get it to Prepend.
Action type: Format value
Format string: What goes here that means to prepend Technical Credits before the existing contents.