If you want the bend text to be indented at the same level as the first line of the fold, you need to add the indentation level to the foldtext layer:
function! MyFoldText()
let indent_level = indent(v:foldstart)
let indent = repeat(' ',indent_level)
...
...
return indent . txt
endfunction
Here, I assume the line txtis your existing foldtext, so all you have to do is add it to the end indent.
But I'm not sure that this is what you want to achieve.
EDIT:
, , , . +. , -, , indent . '+' . txt.
.