Does anyone know how to do this on xtype?

line1
command: xtype
target: ${KEY_ALT_DOWN}
value:

line2
command: xtype
target: ${KEY_TAB}
valye:

line3
command: xtype
target: ${KEY_TAB}
valye:

line4
command: xtype
target: ${KEY_ALT_UP}
value:

Line 1: variable “KEY_ALT_DOWN” is not defined

I want to hold the ALT key. and double click the TAB.

Holding a key down and then pressing another key is not yet supported by XTYPE. One solution would be use an onscreen keyboard and then use XCLICK to simulate the key presses. Or see here: