How to use value of previously show in text box

Support
1

If type is TEXT, I would like to autofills for value of previously select_one show in text box and additional text, Can I do this?

Capture

ie. I want the text box show previous values followed by newly added characters and numbers.
ex. if selected SITE is 'AB01' and SUBJID entered is 'A0001' (therefore full ID is 'AB01-A0001' in text box) How do I set up in 'constraint'?

i'm sorry if I explain, don't understand, my english is not good.

2

This can't be done with a constraint, but can be done. One would need to create a calculate variable, with the concat function.

See: https://docs.opendatakit.org/form-operators-functions/

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3

@CharlieKeyes Thank you for your recommendation, It’s very useful.
Now I can do it, by use the calculation column instead, and show previous values in a once() function.