Let S be an endless string which is a concatenation of every binary counting in succession, starting from zero all the way to infinity (without left zero-padding):

S = 01101110010111011110001001101010111100110111101111...
(from concatenating 0, 1, 10, 11, 100, 101, 110, 111, 1000, 1001, and so far)

Let S' be a set of every sequential group of octets (8 bits) from string S, which can be represented as a base-10 number (between 0 and 255), like so:

S'_2 = [01101110, 01011101, 11100010, 01101010, 11110011, 01111011, ...]
S'_10 = [110, 93, 226, 106, 243, 123, ...]

I'd create an audio wave file whose samples are each octet from S'_10 as 8-bit audio samples, using a really low sampling rate (such as 8000 Hz or even 4000 Hz).
That sound, that particular sound, is what I'd transmit to the cosmos: the binary counting, something with a detectable pattern (although it'd be not so easily recognizable, but something that one could readily distinguish from randomness noise).