The most obvious solution is this: https://openstreetmap.org/?mlat<lat>&mlon=<lon> (replace <lat> and <lon> accordingly). Apps like OrganicMaps/CoMaps and OsmAnd will install a handler for this (at least on Android, not sure about iOS) that will allow you to open such links with the app easily (it usually shows an "open with" menu when you click on such link). Example: https://openstreetmap.org/?mlat=51.477352&mlon=0.0

If you want to specify the "zoom level" (i.e. how much of the map you can see), use https://openstreetmap.org/?mlat<lat>&mlon=<lon>#map=<zoom>/<lat>/<lon>. Example: https://openstreetmap.org/?mlat=51.477352&mlon=0.0#map=19%2F51.447352%2F0.0

There's also osmand.net, which is more flexible. It asks you if you wish to open the location in an app, and if you do, you can use any app (whether it uses/supports OpenStreetMap or not), because it uses Android's native location thingy for that. To use it, do https://osmand.net/map/?pin<lat>,<lon>#<zoom>/<lat>/<lon> . Example: https://osmand.net/map/?pin=51.477352%2C0.0#19/51.477352/0.0