You might be using a link field on your entity type, and if so, you might find yourself in a position where you require the exact URL from the link reference field.

Stick this in your template file and you're good to go:

{{ content.field_custom_link.0['#url'] }}

If you need the title, use this instead:

{{ content.field_custom_link.0['#title'] }}

Be sure to switch out the bolded text above for the machine name of your custom link field.