I don't know about the powershell, but if its created and you require to modify it, use the CLI or the GUI, the below is the CLI way,
(cfg-sync Standalone)(Active)(/Common)(tmos) create ltm monitor tcp test1
(cfg-sync Standalone)(Active)(/Common)(tmos) list ltm monitor tcp test1
ltm monitor tcp test1 {
defaults-from tcp
destination *:*
interval 5
ip-dscp 0
recv none
recv-disable none
send none
time-until-up 0
timeout 16
}
(cfg-sync Standalone)(Active)(/Common)(tmos) modify ltm monitor tcp test1 destination *:8080
(cfg-sync Standalone)(Active)(/Common)(tmos) list ltm monitor tcp test1
ltm monitor tcp test1 {
defaults-from tcp
destination *:webcache
interval 5
ip-dscp 0
recv none
recv-disable none
send none
time-until-up 0
timeout 16
}
(cfg-sync Standalone)(Active)(/Common)(tmos)