A great way to promote your OnlyFans is to post content in subs that are in your niche. For example if you have big boobs, post in a big boob sub.
To find appropriate subs to suit your target subscriber just scroll to the bottom of this page, click I understand to the cookies message, if it appears. Then click on the magnifying glass next to the house image at the bottom of the page and type in a search term, e.g. big boobs. Click search and then just work down the list. Make sure you read each subs rules in order to get your post to “stick” and not have the mods remove it or have it automatically blocked.
It will take a bit of trial and error but if you can get it to work it will lead to a great number of new subs 🙂