Populate first row of a side by side matrix based on a previous question and make it read-only

Hi rarcega , try the following code
Qualtrics.SurveyEngine.addOnload(function()
{
    const prefilledAnswer = "${e://field/EMBEDDED_DATA_FIELD_NAME}" // the embedded data field must EXACTLY match the text of the option choice. Only replace the text EMBEDDED_DATA_FIELD_NAME with the embedded data field of your choice, or use the answer to a multiple choice question by replacing everything inside the quotation marks with ${q://QID1/ChoiceGroup/SelectedChoices}, replacing QID1 with the correct QID.
   
    const container = this.getQuestionContainer()
    const firstSelectBox = container.querySelector('select')
    const options = firstSelectBox.querySelectorAll('option')
    const desiredOptionList = [...options].map((option) => option).filter((option) => option.innerText.trim() == prefilledAnswer.trim())
    if (desiredOptionList.length == 0) {
        alert('No option found for ' + prefilledAnswer + '. Make sure that option is included in your scale.')
    }
    else {
        // set value of select box
        console.log(desiredOptionList)
        const desiredOption = desiredOptionList[0]
        const desiredOptionValue = desiredOption.value
        firstSelectBox.value = desiredOptionValue
        // make select box read only
        firstSelectBox.disabled = true
        firstSelectBox.ariaReadOnly = true // for accessibility
        firstSelectBox.ariaDisabled = true // for accessibility
    }


});
This should give something like the example below, which is what I gathered you were looking for.
Where the first row is read-only/disabled and pre-filled based on a question response or an embedded data field. Another idea I'd consider is not including the respondent at all in the matrix.
I used a likert matrix which may differ slightly depending how your question is configured. Attached is the QSF.
AnsweringForumQuestions.qsf