You can clone the form (the ...) and the Advanced Settings of that form allow you to select a different Site on the same Kentico instance.
If the site is on another Kentico instance, you will have to Sites > Export and export your Form, then Sites > Import on the other site to import the form.
Additional if you just want to DISPLAY the same form on another website, you can adjust the Online Form Widget's Properties (Widgets > Online Form > Properties) to unhide the Form Site field, then you can set the form site property on your actual widget to say "Give me this form that exists on this other site" more or less in the configuration.