I am trying to change the default admin UI URL used in Umbraco to a custom URL. The default URL is simply
<add key="umbracoPath" value="~/asdf" />
From the official Umbraco documentation you can do this one of two ways.
Add this rule to your "/config/urlrewriting.config" file
<add name="adminrewrite" virtualUrl="^~/asdf/" rewriteUrlParameter="ExcludeFromClientQueryString" destinationUrl="~/umbraco/umbraco.aspx" ignoreCase="true" />
Rename 'Umbraco' directory to 'asdf' then change your web.config file as below:
<add key="umbracoReservedPaths" value="~/asdf,~/install" /> <add key="umbracoPath" value="~/asdf" />
The second option is what you have already tried but I think it may be the reserved paths part that you are missing.
Just a word of warning, this has been rather a big issue since about version 4 and from the looks of things on the official community forums, this is still something that works for some people and doesn't work for others.