Here is y code:

static $db = array(
'Bucket1_Link' => 'Text',
);
public function getCMSFields()
{
$fields = parent::getCMSFields();
$fields->addFieldToTab('Root.Content.Bucket1', new TreeDropdownField('Bucket1_Link', 'Choose a page', 'SiteTree'));
return $fields;
}

in the template I use the variable $Bucket1_Link, whhich is id of the page. But I need the URL of the page. How would I get it?
thank you!

Well for TreeDropdownFields you normally save the field as a 'relationship' with a page. Change the

static $db = array( 
      'Bucket1_Link' => 'Text', 
   ); 

To a $has_one

static $has_one = array( 
      'Bucket1' => 'SiteTree', 
   ); 

And slight tweak to your getCMSFields...

 $fields->addFieldToTab('Root.Content.Bucket1', new TreeDropdownField('Bucket1_Link', 'Choose a page', 'SiteTree')); 

Change to

 $fields->addFieldToTab('Root.Content.Bucket1', new TreeDropdownField('Bucket1ID', 'Choose a page', 'SiteTree')); 

Rebuild your database and refresh the page, resave the value then in the template you can now use $Bucket1 as a Page object so you can do $Bucket1.Link

Thanks a lot!

I did like this:

in the control of the page I defined a function:

public function page_url_by_id_bucket1(){
$pageID = $this->Bucket1_Link;
$obj = DataObject::get_by_id('SiteTree', $pageID);
$URL = $obj->URLSegment;
return $URL;
}

and in the template I write:

"<a href="$page_url_by_id_bucket1">Link</a> "

Thats the other way you can do it :) usually in SS we prefer passing around relationships and objects rather then just strings as its alot more flexible.

Watch out in your version - if $this->Bucket1_Link gets unset (eg a client or someone doesn't set it) that function will throw a error as it won't be able to get the $obj then you're calling $obj->URLSegment on it. You'll need to add a check to make sure $obj exists before calling any methods on it.

The only problem with your method is that it doesn't save the selected page from the tree. It always shows the message "(Choose)"