You can write code in  Helper/Data.php to get store url and using data helper call that method in admin page

for example 

Helper/Data.php

    public function getStoreUrl(){
        $storeId = $this->_storeManager->getDefaultStoreView()->getStoreId();
        $url = $this->_storeManager->getStore($storeId)->getUrl("sample/index/test");
    return $url;

    }


In admin page 
In block 

    public function __construct(
		\Namespace\Modulename\Helper\Data $_dataHelper
	) {
		$this->_dataHelper = $_dataHelper;
	}

  
get url - `$this->_dataHelper->getStoreUrl();`


In phtml 

    $block->helper('Namespace\Modulename\Helper\Data')->getStoreUrl();


**For Magento 2.1** 

    echo $this->getBaseUrl().'sample/test/test';