You need to use the Call Microflow action, and here you can pass your String parameter.
https://docs.mendix.com/refguide/microflow-call/#32-parameters
If you are calling this from a page, then you will only be able to pass in an object. In this case, create a new entity and add a String attribute in there. Your microflow will need to read this object to get the String attribute.
I hope this helps.