Function create_promotion_extrinsic 

pub fn create_promotion_extrinsic<Block, C>(
    client: &C,
    at: Block::Hash,
    data: Vec<u8>,
    signer: MultiSigner,
    signature: MultiSignature,
    submit_timestamp: u64,
) -> Result<<Block as BlockT>::Extrinsic, ApiError>
where
    Block: BlockT,
    C: CallApiAt<Block>,

HopRuntimeApi::create_promotion_extrinsic.