Some improvements are needed in the step input area, and I understand that there are long term plans to add features to it, But in the meantime-
While Step Input is active, and, If you have the status line showing in the Key or Score editor, you can tab to the Mouse Position Fields.
Armed with this knowledge, you can create the key command you need via a macro, using the command Focus - Forward. That command is equivalent to hitting the tab key. How many tabs depends on what you have showing in the Tool Bar.
For my setup, hitting the tab key three times places the cursor in the Mouse Position bar field.
So I assign a keystroke to this simple macro:
Focus - Forward
Focus - Forward
Focus - Forward
There is also the command ‘Move Insert Cursor to Part Start’