You could create another microflow as wrapper. A microflow without an input parameter. Use this new microflow as datasource microflow and call the other one as submicroflow. Then you can pass the value you want to have as default.
If you want to pass data to a datasource microflow, you need to put the respective widget in the context of another DataView (or other data widget). Then the microflow can have an Object of that specific entity as an input.
You cannot have primitive types (String, Number) as an input to a datasource microflow, but you could have an entity with a string attribute as input.
What exactly are you trying to achieve?