If you want to pass the parameter to that microflow, it will come from the context (meaning from the parent page). So if you don’t have that page you can create a dataview and then put list in that, the entity of the page can be non-persistent entity which will hold the string dynamic value, if its hardcoded you can directly pass in the data source microflow.
It is not possible to add a string paramter to a microlfow called from a Page having a data view or list view.
You can have string paramter if it used a sub-microflow.
You might want to upvote https://forum.mendix.com/link/ideas/174
Your DataSource Microflow is designed with an input (like I show above) and that is not available as input (nothing is passing the parameter to the DS_Vendor_Outlook. If you give more information what the DS_Vendor_Outlook should do, I could help you. Maybe you want something like this: