@miguel_lopez In standalone mode, you could also use UART which is available on the io0 port (e.g. using an Arduino or similar). Extrusion control comes down to plain G1 E1 commands. If you are looking for a demo, see the pendant controller firmware here: https://github.com/Duet3D/CNC-Pendant-Firmware That is used for axes, but could be changed for extruders as well.
In SBC mode, you can either use the /opt/dsf/bin/CodeConsole utility to send commands to RRF or write your own .NET or Python plugin which reads data from other available RPi pins.
Alternatively, you can send G1 commands using the web interface (HTTP API), via Telnet, or via USB. It's really up to you.
Note that there is no "start extrusion" and "stop extrusion" functionality, so you probably need to send G1 E commands in given intervals while extrusion is supposed to happen.