As far as I know RStudio doesn't officially support running the R interpreter (or R core in Anaconda terms) trough a conda environment.
If you are committed to use conda environments for R, maybe because you are going to heavily use python in your project, I would recommend not using RStudio as your IDE because the integration is not seamless, consider using something more python focused like JupyterLab. Otherwise, consider using a regular R environment maybe with renv for package version management. If your concern is the Python package management side of things, have in mind that using conda envs for Python packages from R (and RStudio) it is well supported.