Learn Farsi: سخنگو به عنوان میانجی بین دو کشور عمل کرد. - The spokesperson acted as a mediator between the two countries.

It connects using the invisible grammatical linker called the ezafe. You pronounce it as be onvân-e miyânji. Whenever you use be onvân-e to introduce a role, you must add this 'e' sound to attach it to the following noun.

Why isn't there an indefinite marker like yek or -i for 'a mediator'?

In Farsi, when you use be onvân-e to state a title, role, or capacity, the noun is treated as an abstract category rather than a single countable item. Therefore, you just use the bare noun miyânji without any indefinite articles.

The English translation says 'acted as'. Which part of the Farsi verb amal kard means 'as'?

The verb amal kard simply means 'acted' or 'took action'. The English 'as' is entirely covered by the preposition be onvân-e (in the capacity of) earlier in the sentence. Farsi separates the role phrase from the verb, giving a literal structure of 'The spokesperson, in the capacity of mediator, acted.'

I recognize goo in sokhangoo (spokesperson). How is this word constructed?

It is a compound noun formed by sokhan (speech or word) and goo, which is the present stem of the verb goftan (to say or speak). Combining a noun with a present stem is a very common way to create occupational nouns in Farsi, literally meaning 'speech-sayer' in this case.