I've found a solution, I've successfully set a message key in my xrp toastwallet address (thanks to tabas for your link, but wasn't helpful)
I want to share with you how I did it:
1) Login into your toastwallet and convert your six words recovery phrase into a secret (
https://toastwallet.com/faq -> Q20)
In toastwallet click on xrp account address and choose button "show secret"
After entering your passphrase the XRP secret is shown, copy that secret (secret starts with s....... and is 29 characters long)
2) Install XUMM app on your smartphone and import wallet from secret. Now you have your wallet migrated from toastwallet to XUMM.
3) Follow the instructions for XUMM wallet spark claim
https://flare.wietse.com/