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.
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.
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.
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.