@Anonymous
With({toggleon:If(ExistingAccountToggle.Value,
Concatenate(DataCardValue3.Text," ","-"," ",DataCardValue12.Text),
Concatenate(WFSAccount.Selected.Name," ","-"," ",WFSAccount.Selected.SalesForceID)
)},
Coalesce(Parent.Default,toggleon)
)
If you put the formula above in the Default property of DataCardValue1.Text then when the form is in New Mode, it will show the first concatenation formula if the toggle is on and the second concatenation formula if the toggle is off.
When the Form is in Edit mode, it will show the previously chosen data. I am not sure that is what you wish but if not, please specify the result you want. In either case, you may wish to disable the card so that the user can't change it as I am inferring from your formula that the result is calculated either way and does not require input into DataCardValue1.