You can keep the entity which holds the textbox value as a helper object ( Have a create or retreive Microflow for this on page load )and create a 1-* association with the other entity. Update the association with the obtained list from the external database. Based on the records associated with the helper Object you can populate the data grid.
Here is what i did according to your suggestion. Both the below entities are non-persistable entities
the search_string is the text box where i can type in text and it has microflow associated with on enter key event.
Below is my microflow where i am commiting the links but those links are not appearing in the data grid