On small boards like arduino micro, NodeMCU(esp8266), or teensy, I cannot run an iota node or wallet. I can only transmit POST requests to a iri server.
Given a seed, and a message, how do I attach it to the tangle using a POST request?
That basically means you have to implement it yourself. Due to low computational power, you will also need an iri node which will do the PoW for you (i.e. you'd have to host it yourself anyway), and in case you want to include a balance in your transaction, I am not sure if you want to do the Winternitz signing on the device itself. In any case, if your message is binary, you'd have to convert it to trytes before sending it to iri.
In other words, it is probably easier to run a service (e.g. written in node.js, or as an IXI module to run inside your iri) on the machine that runs your iri node, which accepts POST requests with message and does the conversion and communication with iri for you. But if you really want to, you can probably do it on your device too. You will also have to do more than one POST request, though.
getTransactionsToApproveto receive two tips to approve.
attachToTangleand pass your transaction string as well as the two tips, to let your node perform PoW and format the final transaction
storeTransactionsand (once completed)
broadcastTransactionsto send your transaction