welcome, and I understand Ctrl+A should work in the URL bar too, guess that's already somewhere on the to-do/bugs list...
But there are alternatives, so no prob
If you don't want to do a quick click with the mouse in the URL bar to highlight the line, click here for a list of the current default commands and key shortcuts ('accelerators' in KM):
Type URL bar on the website and there you are: F6 to select the URL bar line.
And just a little offtopic tip, try a right-click on the Go-button, it has some nice options, as most buttons, even though there's not the usual little arrow beside them.
You can set any of the commands from the above list (and all sorts of macro-commands) on other shortcuts, if you click
Edit > Configuration > Accels
Take the default accels list in your KM program folder as example for syntax:
Those default settings will be overwritten if you define something else for F6 in your profile (Edit>Config...), and of course you could add there
CTRL A = ID_SELECT_URL
but I'm afraid then it wouldn't work anymore for selecting the page ;-)