Boo! A Halloween Code Golf Challenge

Pyth, 19 18 bytes

j" ~BOO!~ "cz]OtUz

Thanks to @Jakube for golfing off 1 byte!

Try it online.

How it works

                    (implicit) Store the input in z.
                Uz  Compute [0, ... len(z)-1].
               t    Remove the first element.
              O     Select an integer, pseudo-randomly.
             ]      Wrap it in an array.
           cz       Split the input string at that point.
j" ~BOO!~ "         Join the split string, using " ~BOO!~ " as separator.

GML, 91 bytes

s=get_string("","")
show_message(string_insert(" ~BOO!~ ",s,irandom(string_length(s)-2)+1);

Simple enough - get a string, insert the substring into it, output the string. Done.

Python 3, 60 bytes

s=input();n=1+hash(s)%(len(s)-1);print(s[:n],'~BOO!~',s[n:])

Note:

The modulo of hash() will be uniformly distributed over the length of the string. If you think that's bending the rules, note that because of python's hash randomization, this is actually random: repeated executions with the same input will give different results.