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

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.

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.