Demand Gen can introduce a product before someone searches for its brand. Whether that work improves this account's results is a testable hypothesis, not something branded-search movement establishes by itself.
Start with a buyer question, supported creative and a useful destination. Define the audience, placements, budget boundary and measurement window before launch. Use the current account's eligibility and platform documentation rather than assuming a fixed reach or audience-overlap percentage.
Review the whole buying path. A rise in branded searches may be consistent with demand creation, but seasonality, promotions and other media can move it too. A brand-search ROAS increase does not assign the later revenue to Demand Gen.
Where feasible, design a compatible control or holdout. Otherwise describe the result as an observed association and retain its limitations. The allocation should follow the actual economics and learning needs; a fixed percentage of spend is not suitable for every account.
Take the idea further
Plan the Demand Gen test