Signature: encoding a message - transaction to a point of curve y^2=x^3+7 and Bitcoin Core
To sign a transaction (message) in Bitcoin system, you need to encode the message to a point of the curve y^2=x^3+7


Perhaps you're thinking of ElGamal encryption, whereby you do need to select a point on the curve which represents your plaintext.

However for signing, Bitcoin uses ECDSA, and this has no such requirement.

If you're interested, the code which implements ECDSA signing is in secp256k1_ecdsa_sig_sign(). For example, this is the code which implements step 6 from the Wikipedia link above.