Most academics are open to meeting prospective postdoctoral candidates or future collaborators (even if they don’t currently have funds to recruit you). Many plan for the long term. The best thing to do is contact them ahead of time by email to schedule a meeting. It’s best to meet in person if you can but videoconferencing is also okay. They’ll be interested in whether you can communicate well and work with their research group in addition to potential research projects. If you are visiting their institution while travelling for a conference in the future or planning to apply for fellowship funding that you are eligible for.
You can also offer to give a seminar when visiting their department. They can offer support to host you be do not expect this. If you plan to visit, contact them long in advance in case they need time to organise your visit.
Please be patient and respectful, they are very busy and may not get back to you straight away. It’s helpful to mention any common connections you have including working on related projects. If you can, you should mention that you know their collaborators or have met them before at a conference. If their collaborator has recommended to contact them, you should include that when you introduce yourself but you can still contact them directly rather than getting someone to do it on your behalf.