The register procedure is empty.
How do I add the required code? The file is created automatically.
This is probably because you did not register your own components - yes, there are many registration calls throughout the LCL...
We could provide more specific help if you'd show your units and what you already have. So we have to stirr in the fog...
Anyway: Suppose you want to create a new component TMyCombobox. You have a unit "mycomboboxunit" in which you declare the component and do the implementation. In order to register the TMyCombobox you must add a public procedure "Register" to the unit, and you must call RegisterComponents (in the classes unit) with the name of the palette for the component and with the class to be registered. Basically, these additions should look like this:
unit MyComboboxUnit;
interface
...
type
TMyCombobox = class(TCombobox)
...
end;
procedure Register;
implementation
{$R mycomponents_icons.res} // see below
...
procedure Register;
begin
// This adds TMyCompbox to palette "My Components".
RegisterComponents('My Components', [TMyCombobox]);
end;
end.
The square brackets in the RegisterComponents call indicate that you can register several components in the same call on the same palette tab. Suppose you also have a TMyEdit then you could register both by calling
RegisterComponents('My Components', [TMyCombobox, TMyEdit]);
This is the reason why the registration of large collections of components is often moved to a separate unit dedicated to registration.
The new component will have the default palette icon. If you want to have your own special icon you must draw a bitmap (ideally a PNG for better transparency, but a BMP is fine, too). The bitmap must have size 24x24 and must have the name of the component (in our case: TMyCombobox.png'). For better quality on high-dpi screens you should optionally create also images at 36x36 and 48x48 pixels (150% and 200%); their file names must have an appended _150 and _200. Then compile the application LazRes (in folder "tools" of your Lazarus installation) and use it to create a resource file from the image:
lazres mycomponents_icons.res TMyCombobox.png TMyCombobox_150.png TMyCombobox_200.png
The resource file (the first parameter of the lazres call) must be linked to the package in the registration unit, as shown above in the {$R ...} directive.
Please study some simple third-party packages to learn how things work together. Packages with single components are for example "chemtext" or "colorpalette" on CCR (or install via OPM).