You can have a look at this blog: https://medium.com/mendix/create-your-own-way-of-filter-by-npe-non-persistant-entity-and-microflow-4ffd11e52fb0
This will be a good start for your implementation.
You can use the helper entity with the search string in a database retrieve with the function contains.
For more info, see the documentation: https://docs.mendix.com/refguide/xpath-contains/
Let me know if this helps.