First of all, in the WiiM app, under Browse/ Amazon Alexa, link your Alexa account. Once that is complete, your WiiM device should appear in the Alexa app under Devices/Echo&Alexa with the name “WiiM Pro”.
You don’t say which music service you’re using, but you should be able to ask your Amazon Echo “Alexa, play music on WiiM Pro”.
Alternatively, if your Echo is already in an Alexa smart home device group, you can set the WiiM device as the default speaker for audio for that group by setting the Wiim as preferred speaker under the group’s Speaker setting, choosing the option to always play on that without having to specify the group name. If your echo isn’t already in a group, then in the alexa app under devices, choose add (+) , then add group, add room or device group, choose your echo and give the group a name like ‘living room’ or whatever you choose. When the group has been saved, select it on the devices page then edit, scroll down to the speaker setting and select that to specify the Wiim device as your preferred group speaker. After that, asking that echo “Alexa, play music” will play by default on the WiiM.