Trying to write func("abcd") -> "abbcccdddd"

[omm]

For passing a parameter 'abcd' to function tag, the following result is expected tag("abcd") -> "abbcccdddd"

Now I am trying to append these values to '' empty string but, not getting the result

def tag(s):
    count = 0
    res = ''
    for val in s:
        count += 1
        for i in range(count): 
            res.join(val) 
    return res # Expecting Toommm


print(tag("Tom"))

Am I doing something wrong here?
I have tried to print(i, val) and that seems to print correctly the expected characters.
But, could not append the same to empty string and output it.

            res.join(val) 
    return res # Expecting Toommm

Thanks,
Om