Hi @Knawl ,
Firstly Passing the text from a label to Power Automate:
Assuming that the trigger of the flow would be Power Apps, in the Power Automate, against the Task Title, You need to select "Ask from Power Apps" from the right navigation as shown below.

Once You Select that, the Flow would be expecting a parameter while triggering from the Power Apps end. So your flow trigger in Power Apps should be like below:
Yourflowname.Run(label.Text)
The Label.Text would provide the Value in the Flow.
Secondly Assigning to users from SharePoint List:
If its going to be many users, Please let us know how the users are populated in the SharePoint List.
Is it a Look Up List ?