Hi,
As per my understanding, you want to get and set the values of the custom Option Set field in the portal for the Title in the Case Table section.
For which, I have shared the screenshot below from my test scenario, you can Select from the Options in the Topic section to change the Title.

You have to follow below steps:
1. Navigate to the js File of the respective web page,
> click on the edit code (You have to login in Vs code)
2. Sync the Vs Code
> tap the Sync button on the Portal design studio after edit code
3. Find the Element id
> Logical names are Id for Element if not then you can achieve this with inspect
Initially you need write some lines in js for the respective page, here’s a screenshot for your reference.

You have to use the field id for your own scenario, (Logical names are the id for Element in HTML), you may refer to the below code to be added in the js file.
$(document).ready(()=>{
//getting the title text
titleText = $("#title").val()
//geting the Topics
caseTopics = $("#crb24_topicexamples")
//On option change
$(caseTopics).change(()=>{
//getting the value of the Option
caseTopicValue = caseTopics.val();
//getting the text from Value
//if blank then set the Onload Value of Title
if(caseTopicValue == '')
{
$("#title").val(titleText)
}
else
{
caseTopicText = $(`#crb24_topicexamples option[value=${caseTopicValue}]`).text()
//setting the text as value in title field
$("#title").val(caseTopicText)
}
})
})

Hope this was helpful.
Thanks!