I have done it with Multibit, let me try with electrum now, but it will be another wallet address.
So here I go:

Message:
Hi BItcoinSupremo here
Using electrum 2.5.4 on linux
trying to stake my address below.
Thank you

Address:
18jwZu4mWeGsZJfyua8ds68wzg5gmZiv49

Signature:
IPHHz/z/2RDSZQ+2lLac39B+xc4y9YOP5/qh4U4kGxzed+d+dNmpVautWoLdsJ+0WPAQvr3tHpQd0Xu1/+dpl2A=

After doing this Electrum told me, Signature verified. Hopefully I have done it right this time.