Append JSON to variable

How do I append values to a variable without it converting to string?
I need to have a list of json, I'm sending this to my endpoint:

{
  "input": {
    "messages": {{Chat history}}
  }
}

{
  "input": {
    "messages": {{Chat history}}
  }
}

{
  "input": {
    "messages": {{Chat history}}
  }
}

{
  "input": {
    "messages": {{Chat history}}
  }
}

tried to use the "custom" option for the variables but I still get a list of strings in the Chat history variable