Filter list values in subform based on list value selection of another field in same sub form

[Remsky]

I am hitting a wall here... I have figured out so much about Access (2019) since I started a week ago but I'm truly stuck.
I have a subform in which I have 2 combo boxes. The first combo box should operate as a filter on the second combo box.
See attachment for DB Structure of relevant tables.

The main form is linked to tblMutKop (header information regarding the underlying financial entries)
The sub form is linked to tblMutDet (all individual financial entries).
First combo box: list values from tblSTAM_Rel (=customers table).
Second combo box: list values from tblMutKop.

Question: how can I, in the second combo box, only show 'Datum' and 'Factuurnummer' of MutKop records with the in combo box 1 selected 'RelCode' (which is NOT the 'RelCode' from the main form)?

SELECT 'Datum', 'Factuurnummer' FROM MutKop WHERE 'RelCode' = <RelCode combo Box 1>

How can I make this work?

[June7]

This is known as cascading combobox/listbox and common topic in many forums.

SELECT Datum, Factuurnummer FROM MutKop WHERE RelCode = [combobox1name];

Then you need some code in VBA to requery combobox.

[Remsky]

This is known as cascading combobox/listbox and common topic in many forums.

SELECT Datum, Factuurnummer FROM MutKop WHERE RelCode = [combobox1name];

Then you need some code in VBA to requery combobox.

Thanks, I was so close without even knowing it!
And not knowing the exact terminology made it more difficult as well. Sorry for that...

After setting up the requery thing it's working now as a charm!!