I used BayesSearchCV from scikit-optimize for my manuscript. I searched online but I do not find a proper citation for it.
One option is:
Head, T., Kumar, M., Nahrstaedt, H., Louppe, G., & Shcherbatyi, I. (2021). scikit-optimize/scikit-optimize (v0.9.0). Zenodo. https://doi.org/10.5281/zenodo.5565057
If you used a older version of the package, change the citation accordingly, e.g. if you click on this link and look at v0.8.1, the citation is:
Head, T., Kumar, M., Nahrstaedt, H., Louppe, G., & Shcherbatyi, I. (2020). scikit-optimize/scikit-optimize (v0.8.1). Zenodo. https://doi.org/10.5281/zenodo.4014775