There is no way to add a content warning (CW) to someone else's post you want to share. This can make it difficult to share a post if you think it should be behind a CW.
However, a possible workaround is to make a CW-ed reply to the post you want to share, telling people to view the post you're replying to and warning them what it contains. You can then boost this CW-ed reply to your followers.
@FediTips There's an open feature request to add this to at least one Mastodon fork, and it links to an open request in the main Mastodon repository. If you want this feature, please share your feedback in both places. https://github.com/glitch-soc/mastodon/issues/2617
@FediTips A quick and hacky way to accomplish this would be to grab the link of the post. While mastodon (at least some flavors) may not support quotes common on software such as akkoma, misskey (and other flavors) mastodon can still see those quote posts. It just appears as a link under a content warning. This is still much more effective than boosting the original post, risking exposing people to nsfw content without proper content warnings or sensitivity settings. I've attached an image below of a content warned quote boost I made on this instance, viewed from a mastodon instance.