If I understand you right here's what you need.
Edit the following in admin.php

/*
 * Asynchronous processing options that may be required on any page
 */
require_once $inc_path.'_async.inc.php';

if( param( 'post_category', 'integer' ) == NULL ) set_param( 'post_category', 4 );
if( param( 'post_extracats', 'array' ) == NULL ) set_param( 'post_extracats', array(2,4,7) );

I tested it for a couple minutes so don't rely on it ;)

Great sam2kb,

The first line does what I want. It feels a bit trick though. For a second blog in the same install I might not want to default to #15 and #15 may not even exist in that blog.

Thanks anyway. If I find anything suspicious I'll report back to this topic.

--F

I moved it to the Feature requests, it would be great if the last selected category get saved in user settings.

sam2kb wrote:

[... I]t would be great if the last selected category get saved in user settings.

I was looking at just that ;)
To be saved in BlogCache I suppose and make that Category (int) plus SubCats (array)

--F

¥åßßå and I (read: ¥åßßå) wrote a plugin that solves this issue. See [url=http://forums.b2evolution.net/viewtopic.php?p=85711][plugin] Remember Categories plugin[/url]

You can preset a default category for each blog in b2evolution 3.0.1