DistinctCount for relatedtable with additional filter In powerBi, a column is created with below formula, and it is working good.

activedays = COUNTAX(RELATEDTABLE(ActivesInactiveData),ActivesInactiveData[Surface Date])

However, I want to add a filter some thing like

ActivesInactiveData[Topic]&lt;> null

Could you please advice on how to do so, most of the examples on web are using on relatedtable etc - where relation is already build, could you please advice/direct to me what I could find an example of adding filter to relatedtable or filter data from a unrelated table based on field like ID column etc.

Here there is a relation user ID in current table has more than 1 rows in ActivesInactiveData table.
I would wrap your expression in a CALCULATE function, so something like:

activedays = CALCULATE ( 
    COUNTAX ( RELATEDTABLE ( ActivesInactiveData ) , ActivesInactiveData[Surface Date] ) 
    , ISBLANK ( ActivesInactiveData[Topic] ) = FALSE )


I assumed [Topic] is text, so NULL will converted by DAX to BLANK().