[User 224999]

Well, just gave it a quick shot but it doesn't work.
Seems that the Keyboard popup entry is missing, or the getdirectory error ?.

You must use the same syntax as the normal skin settings, so providing the values in your skinsettings.xml file
That way you can also set the defaults with setting the optional attribute constantdefault="condition"

So, in skinsettings.xml:
<setting id="MyConstant" value="200" label="200" condition="" icon="" description="" constantdefault="true" />
<setting id="MyConstant" value="300" label="300" condition="" icon="" description="" />

And call the script:
RunScript(script.skin.helper.service,action=setskinconstant,setting=MyConstant,header=Width for Panel)

The includes file will be auto generated based on user selection and/or the defaults, no need for you to supply it.

If you want I can also add a prompt option so you supply some values and a way to enter manually, something like this:

<setting id="MyConstant" value="||PROMPTSTRING|||" label="300" condition="" icon="" description="" />
<setting id="MyConstant" value="||PROMPTNUMERIC|||" label="300" condition="" icon="" description="" />