I have a text entry matrix table with one row and two columns.
I want to populate column one with "CP104" and column two with "A"
${q://QID18/ChoiceTextEntryValue/5/1} would pipe "CP104"
${q://QID18/ChoiceTextEntryValue/5/2} would pipe "A"
So, I use the URL with the following parameters:
Q_PopulateResponse={"QID18":{"5":{"1": "CP104"},"5":{"2": "A"}} }
The above does populate column two with "A", but does **not** populate column one with "CP104"
If it matters, there is a RegEx custom validation for column one in the survey, but "CP104" passes the validation.

https://www.qualtrics.com/support/survey-platform/survey-module/survey-flow/standard-elements/passing-information-through-query-strings/#faq-194