What was the message Yonah was to call upon it? The pasuk does not seem to describe it. And so says Rashi, that he should call upon it Hashem's keriah.
Metzudat David makes it even more clear, that He did not spell out just yet what to proclaim upon it. And a bit later, he works this fact into the narrative. Yonah fled outside of Eretz Yisrael, where prophecy does not reach. This way, he would avoid the possibility of receiving another message from Hashem, which would spell out the particular prophecy.
Radak also maintains that it is not written here just what the prophecy was to be. Though it seems that Radak does think that the prophecy was spelled out at this point to Yonah. We just have to deduce from the words that Yonah eventually says just what the prophecy was. And indeed, this certainly seems true later on in Yonah, where Hashem again tells him to prophesy and he does, but with words not spelled out earlier in the text. Thus, in perek 3:
Here, the pasuk just refers to "the proclamation that I bid thee". And we deduce just what the proclamation was that Hashem commanded by looking two pesukim later. And this the meforshim note. And indeed, by saying הקריאה, Rashi is certainly looking ahead to this ambiguous pasuk in perek 3 and stating that whether it says את הקריאה or not, it means the same thing, and both ambiguous pesukim should be resolved via Yonah 3:4.
I look at the second pasuk in Yonah and I see a pasuk ripe for ambiguous parsing. "Ki" can either mean "because" or "that". Rashi et al. adopt the former, such that he merely it to proclaim something upon it, and such that the second half of the pasuk is the reason for that unspecified proclamation.
However, if we read "Ki" as "that", then we could have the following translation instead:
Thus, the second half of the pasuk is the content. We see later that the proclamation is for the overturning of Ninveh, וְנִינְוֵה נֶהְפָּכֶת.
To me, this calls to mind parshat Vayera, and the overturning of Sodom. Just as in Ninveh, their wickedness came before Hashem, we see regarding Sodom that:
and in the next perek, regarding the destruction that results:
|כה וַיַּהֲפֹךְ אֶת-הֶעָרִים הָאֵל, וְאֵת כָּל-הַכִּכָּר, וְאֵת כָּל-יֹשְׁבֵי הֶעָרִים, וְצֶמַח הָאֲדָמָה.||25 and He overthrow those cities, and all the Plain, and all the inhabitants of the cities, and that which grew upon the ground.|
Thus, the message was Divine recognition of the wickedness of Ninveh, and associated with it, an overturning of the city. And so, while Yonah 3:2 gives us solid basis to interpreting it as an unspecified prophecy, I believe there is much merit to the alternative parse I proposed.