How to represent foreign key in an Extended ER Model in Chen notation?
I think I found the answer, it is more or less not possible and anyway not intended in an ERM. There is a small workaround left in using double underlining.
There is simply no FK to mark in an official ERM. See http://faculty.juniata.edu/rhodes/dbms/ermodel.htm:

E-R Model is not SQL-based. It's not tied to any particular logical implementation of a DBMS. It is a conceptual and semantic model, which attempts to capture meanings rather than an actual implementation.


DO NOT THINK OR START WITH TABLES--YOU WILL BE MISGUIDED ON RELATIONSHIPS AND SOME ATTRIBUTES.

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Foreign key: term used in relational databases (but not in the E-R model) for an attribute that is the primary key of another table and is used to establish a relationship with that table where it appears as an attribute also.

And in https://en.m.wikipedia.org/wiki/Weak_entity:

In a relational database, a weak entity is an entity that cannot be uniquely identified by its attributes alone; therefore, it must use a foreign key in conjunction with its attributes to create a primary key. The foreign key is typically a primary key of an entity it is related to.

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An identifying relationship is one where the primary key is populated to the child weak entity as a primary key in that entity.

Thus, the weak key attribute is not equal to a foreign key, it is simply a primary key of another entity needed to reach an entity’s uniqueness (a weak entity is effectively the interim entity that you need to dissolve an M to N relation, using the PKs of “both sides” as attributes; this is out of my course script). Thus the dotted line is not the format for FKs.
Result:
Then, only double underlining remains as an unofficial convention which is up to now used at the university. And mind that showing the FKs is not needed anyway, a named relation implies it and should not explain it, an ERM is not about FKs; do not think in tables.