I'm afraid it may not be possible.

In order to fill a Polish prescription abroad, the doctor must indicate in the system that it is a cross-border prescription and it must be issued in the form of an e-prescription. Currently, such a prescription can be filled in Croatia, Finland, Estonia, the Czech Republic and Spain.

To buy Triamhexal in Germany you will need to visit a German specialist.

As in Poland, also in Germany the drug is registered in two doses - 10 mg/ml and 40 mg/ml.

Since the drug is not approved for marketing in Poland, it is possible to apply for its import as part of targeted import. The procedure consists in importing a drug for a specific patient at a special request of a doctor, with the consent of the Minister of Health. In such a case, the drug is delivered to a pharmacy in Poland near your place of residence.
You can read more about the procedure here:

https://www.gdziepolek.pl/artykuly/import-docelowy-jak-sprowadzic-lek-z-zagranicy

Here you can learn about the procedure on the basis of another drug:

https://www.gdziepolek.pl/opinie-i-pytania/109620/witam-mam-pytanie-czy-jest-ktos-z-panstwa-w-stanie-przedstawic