Hi Alex,
Meaning 1: 'Forthcoming' and 'upcoming' both refer to something that will happen in the future.
Examples: There is a forthcoming/upcoming concert next week in the park.
The forthcoming/upcoming board meeting is extremely important.
However, 'forthcoming' has two additional meanings, while 'upcoming' does not.
Meaning 2: To be available or provided when needed
Examples: Emergency aid was not forthcoming from foreign countries when the earthquake hit. (OR) When he was in debt, help was not forthcoming from his family.
Meaning 3: Willing to provide information
Examples: She was not forthcoming about her involvement in stocks trading. (OR) The teenager was not forthcoming to his parents about sneaking out of the house.
Hope this was useful!