The format for
Where the range is the line or lines you want to move and the address is the target. Note that the moved line(s) will actually be placed on the line below the address.
So you're looking for a range of "mark a" and an address of "the current line". You can find all the special names for ranges and addresses here:
:h :range aka
That tells us that marks are denoted with a
' followed by the mark name and that the current line is denoted with
That gives us...
Or, abbreviated a bit...
By the way, you mentioned
:m'a'b. That actually won't do what you suggest. When you don't specify a range the current line is implied. And when you specify two marks as the address the second one is (apparently) ignored. So that command's full form is
:.move 'a and it moves the current line below the line marked as