In order to use Siri to voice control your WiiM Mini, you need first add it into Apple Home App as an accessory. 

Add WiiM Mini into Apple Home App

Here is the step-by-step guide

  1. Open the Home app and tap 'Add Accessory or Scan Accessory' or '+' icon in top right corner.

  2. Tap yellow colored "More options...?". (Without selecting 'Scan a Setup code' or 'Hold iPhone Near Accessory'). 
  3. Tap your device which is listed under "Select an Accessory to Add to you".

Please select the yellow "More options...?" in step 2. 

There is also a video tutorial here for your reference. Please check it out. 

If you need your Airplay code, please check this article