The seeds must be placed and plinthed in the office of the island. However, you can only add one seed per island. The question mark in the information on the fertility of an island stands for a freely selectable selection of seeds that cannot yet be planted on the island.
If the seed is placed in the office, it can be activated by clicking with the left mouse button. Then the farms can be set.

The Island I am on now does have the wheat icon and I also added the seed to the harbor office, but I still don't see the farm in the building menu for some reason.

https://gtm.you1.cn/sharedfiles/filedetails/?id=2916860156

I think that you have not yet settled patricians and therefore cannot sow wheat yet. Additional seed for wheat is planted on this Island not needed.

Ah okay, that must be it then. I have not settled in the orient yet, thought I first use the northern Islands and expand there.

Thanks :)

Nice if I could help.