Hi Ingar,
on SX900 there is no hardware button for "Talk" function. But you can assign either a Foot Switch (connected on one of the two Foot Pedal jacks) or one of the six "Assignable" buttons (below the display) to "Talk" function:
Press "Direct Access" button first, then press the connected Foot Switch OR press an "Assignable" button (A - F). This will open the "Assignable" display. Here you can assign the function "Talk On/Off" (Category "Mic") to the desired Foot Pedal (1 or 2) or to an Assignable button.
Please refer to the chapter "10 Controller Function Settings" in SX900 Reference Manual (pages 111 - 116).
Note: Foot Pedal settings and Assignable Button settings can be memorized in Registrations (and therefore this settings can also be CHANGED by Registrations, if the corresponding boxes were checked when a particular Registration was memorized).
Hope this helps!
Best regards,
Chris