You can apply a "User Template Extension" on "Source Code Assist" by pre-defining a header, which will be inserted before the actual keyword (code template) when using Content Assistant Keyword Proposal functionality.
To apply a individual header text can be can be added to any provided keyword template:
In the Eclipse IBM/Informix 4GL Development perspective open a file.
In Eclipse go to Window > Preferences >Eclipse4GL > Code Templates.
Choose a provided keyword from the upper list.
Enter a text (usually a comment) in the middle area edit section.
Click Apply / Apply and Close.
Activate option in preferences
Make sure you have activated "Apply User Template Extensions on Source Code Assist" in Preferences if you want to use them by deafult.