How to define child node in Firebase by IONIC 3

I’m working a project with Firebase Realtime Database and Ionic 3, but I have a problem with child node.

for example: My child is “LkuCxf041hHfRkn3tiq”, how to set this child to “test”?

My code is:

sendMessageModule(){     this.db.database.ref('/tasks').child(this.uid).push({       taskM: 'onM'     })  

I tried it that way too, but I didn’t succeed:

sendMessageModule(){     this.db.database.ref('/tasks').push({       taskM: 'onM'     })  

Thank you all!