Many features allow you to forward to an external number using the "External number" entry box on the destination selector. You can play DTMF to the destination if this is required to specify a particular extension. For example, to dial "2125551234" and play DTMF "123" when answered, enter "2125551234x123" in the external number entry box. Anything after the "x" is taken to be DTMF to play.
You may need to delay the DTMF depending on the menu the far end plays. You can do this by adding a "w" to the string specified. Each "w" creates a delay of 0.5 seconds. You may specify as many of these as you wish. For example, "2125551234xwwww1w123" will dial 2125551234, then when answered waits 2 seconds, plays a 1, waits another 0.5 seconds, plays 123, then connects the caller. Some experimentation may be required to find the correct combination of DTMF and waiting.