@Override
			protected void actionPerformed(NavigatableActionContext context) {
				ProgramSelection highlight = context.getHighlight();
				highlight = new ProgramSelection(highlight.union(context.getSelection()));
				setHighlight(context.getNavigatable(), highlight);
			}
Create the actions and add them to the tool.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/highlight/SetHighlightPlugin.java