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

You should interact with the component to execute saveInto expression

You can save the value by using a!save() at another sail component and that should be have user interaction

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)
)
)
}

)
)

Why not just set the value in a local and use that? It seems like a fixed value; if you want to just display a value, place it either in an ri or a local and then set it as the display value in the field. There's no need to use the saveInto on a read only text field.

There is a sail recipe in the documentation. There the default value for fields is set in the data input on the user input task. forum.appian.com/.../SAIL_Recipes.html

Maybe we need more detail here, but the only thing I can assume is that OP is attempting to push an arbitrary on-form literal value ("testValue") up into a Rule Input - I suppose to be passed back into process? Not sure.
If this is the case then I think @chetany nailed it, with the suggestion to store the literal value in a local variable, display the local variable in the read-only text field, then push it to the Rule Input on button click. I'd note here that it may be a better practice to declare the literal value in the ACP definition on the user input task; but there may be special use cases that don't allow this for whatever reason.

A user can only update a variable by interacting with a component that has the variable in the component's saveInto parameter.