Nested Form

Q1.
Background: I have a form for building time series model, in that there is a selectbox with options manual or auto for hyperparameter tuning.
Requirement: I want to create a nested form that is when i choose manual an another form should open so that i can put values for my hyper parameter.

Q2. Can i give default values for a selectbox(this case auto as default selection)

I also interested in a usecase with nested form, did you find a solution to that?