If you are not using the block editor there is only one way that I would do this and that is to use ACF Pro and create an options page and that is where it would be entered.
There are topics here about getting blocks that use ACF from other posts. I do not use the block editor and I cannot tell you how to do this. Searching for this should turn up some recent results.
If you are not using the block editor the you can either use an options page as above or you can supply the post ID to get the values from the other post if you know the post ID
get_field('your_field_name', $post_id);
see https://www.advancedcustomfields.com/resources/how-to-get-values-from-another-post/
