I've gone through these
http://dotmeta.com/expressionengine/guide/ee-2101/add-ons/channel/channel_form/index.html
http://dotmeta.com/expressionengine/guide/ee-2101/add-ons/channel/channel_form/examples.html
https://docs.expressionengine.com/latest/channel/channel_form/index.html
And i don't see a way to tell the {exp:channel:form ...} which Guest user account to use when creating a new entry.