Assigned To Field - Power Automate

In the trigger "For a selected item", you can add an input like the below:

Below steps worked for me, please check if it's working for you:

• Create a flow with trigger "Manually trigger a flow" trigger. And select email option.

• Add "Create approval" action and set email property as assigned to in this action.

• Set other values in above action as per your need.

• Run the flow and pass email address of the user you want to set as assigned to. And click run now.

• Assigned user will get email with approval message card in that as shown below:

Note: The email address you pass here should be the Microsoft 365 licensed account user.