I have a read only text field with value pre-populated. How can I get the displa

As correctly said by @nageswararaoa, the value will not be saved if the user does not interact with the SAIL component. There is another solution. Specify the default value in local variable in load. Then save that variable in rule input in the buttonLayout. So that when the user submits the form by clicking on "Submit" button, the value will get saved into the rule input automatically. In this case, the user need not interact with the text field. Your code will be like this:
load(
local!tv: "testValue",
a!formLayout(

firstColumnContents: {

a!textField(
label: "label",
value: local!tv,
readOnly: true(),
saveInto: local!tv
)
},

buttons: {
a!buttonLayout(
primaryButtons: a!buttonWidgetSubmit(
label: "Submit",
saveInto: a!save(ri!tv, local!tv)
)
)
}

)
)

As correctly said by @nageswararaoa, the value will not be saved if the user does not interact with the SAIL component. There is another solution. Specify the default value in local variable in load. Then save that variable in rule input in the buttonLayout. So that when the user submits the form by clicking on "Submit" button, the value will get saved into the rule input automatically. In this case, the user need not interact with the text field. Your code will be like this:
load(
local!tv: "testValue",
a!formLayout(

firstColumnContents: {

a!textField(
label: "label",
value: local!tv,
readOnly: true(),
saveInto: local!tv
)
},

buttons: {
a!buttonLayout(
primaryButtons: a!buttonWidgetSubmit(
label: "Submit",
saveInto: a!save(ri!tv, local!tv)
)
)
}

)
)