By using the User Control Editor (UCE).
Steps:
- Open the UCE: File UserControlEditor.exe under the GeneXus installation directory.
- Create or edit the User Control. If you are creating a UC, it must be saved under the User Controls directory, which is also under the GeneXus installation directory.
- Define its properties and behavior (Properties, Events, JScript Runtime Render, xsl Designer Render). Save the UC.
- Close GeneXus (if it was opened), and execute GeneXus.exe /install (by command line).
- Open GeneXus as usual.
The new UC will be added to the Toolbox, under the User Controls section.
More information: What is a User Control?
Samples: User Controls samples