Hello @IRieger,
I think the requirement you want to implement is that external users can get the plan name specified in it by typing team name in MS Form. It is not feasible to add the task directly through this way, because the Connector MS Teams does not provide APIs the plan included in the separate team.
However, assuming that each of your separate teams contains one or more plan boards, which are not updated frequently, you can add the team plan to your MS Form options when an external user enters the team name in the MS form. At the time, you can choose the team to which he belongs, just like the screenshot below.

You need to create a SharePoint list, which stores the team name and plan name. When you create the team task to which the external user belongs, you will find the corresponding plan name. Then the flow will get the Plan ID of the corresponding plan name in the plan list. Plan id add task. About adding the next step by looking up the corresponding plan name, you can refer to my last reply.

In this case, the user does not have to fill in the plan name, only need to know the team name, but there is only one designated plan board in each team.
Of course, this is based on the fact that your team is limited and not updated frequently. If your team and plan name change, you need to make changes to the TeamList data in SharePoint. I think the only inconvenience is that these are manual. Operational.
I hope this method can help you.
Best Regards,
Community Support Team _ Lin Tu
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.