please

RouterOS already supports booting via DHCP or bootp just set or follow the instructions here https://wiki.mikrotik.com/wiki/Manual:RouterBOOT

What do you need to do that can't be done with these options?

@deadcat is right, with DHCP + netinstall and/or flashfig is how to install ROS over a network.

If you're looking for an iPXE server, different question/needs: ROS support TFTP, so possible. While not exactly easy, at a high level: you can upload the iPXE server, then use a few /ip/tftp/add ... commands so ROS's TFTP server know to server the uploaded boot files, then use the ROS DHCP Server to set the boot image for clients (on the right interface etc. etc.).

Sorry I did not specify.
I explain better
I was asking for ipxe chainloading via dhcp
To do this you need that dhcp server has support for class-based options.
So the request may sound like this: add class-based options to dhcp server to support ipxe chainloading

Fair enough, that makes more sense now.

But let's say class-based options were support... You'd next run into that it isn't so easy to use ROS to serve a file via HTTP/S with some boot image that iPXE configurations might want to use...

Of cource the chain-loaded iPXE would issue a 2nd DHCP request and this request must be answered different based on the DHCP user class.
The answer to the 2nd DHCP request would point to a real HTTP server (not the MT router).

Me too would like to see that: DHCP 'option set' based on DHCP user class.

Can I have it please

Assuming that for that iPXE booting you have a separate server, it is already possible using DHCP relay.
Configure your central DHCP server with the options and facilities you need, that can include not replying to requests that are not from iPXE clients.
Then you can configure a local DHCP server on each router with a small delay threshold, and when the central server does not reply it will serve the clients using the MikroTik DHCP server.