You can select your default Slack channel in your settings. This will help you to select the right channel from the list faster. Follow these steps:

STEP 1: Navigate to the settings by clicking on your name on the top right of the app and choosing Settings.

STEP 2: Click on tab Account and then Slack on the menu on the left.

STEP 3: Select the wanted channel under Customization, from the list. Click then Save.

NOTE: This article tells you how to get a private channel shown here as well.