I figured a workaround could be to runs some client-side JavaScript in a Node that automatically sets focus on the first field on the page, but I couldn’t find a Node that allows you to inject client-side JavaScript.
One solution would be to run a patched version of the platform-ui, but that seems overkill just for form focus.
There is a node in the Marketplace called Client Script Auth Tree Node, that executes a client-side browser Javascript. Please note that nodes from the Marketplace and not officially supported by ForgeRock.
More detailed notes on client-side scripting which may provide some answers for your request are provided here:
I hope the information provided here assists in your efforts.