Add a custom text/button on the start and preview screen

[Refurb]

Hi there!

I've made plugin and tried to display a Help-button on the right top corner of the screen when on the start screen and in the preview mode (just after the capture). Whenever people click on the button there is a telephone number of me so people can always text me when there are problems at the party.

Is this possible with a plugin or can you tell me where I can do this?

Thanks!!