These words can be slippery. Here’s how we write (or abbreviate) them. (If it’s not on this list, defer to the AP Style Guide.)
dialog or dialog box (the UI element) drop-down (noun, adjective), drop down (verb) To name
From name
Reply-to name
Subject line
Cc, Bcc
emoji (singular and plural) login (noun, adjective), log in (verb) Pop-up (noun, adjective), pop up (verb) signup (noun, adjective), sign up (verb) small and medium-sized enterprises (SMEs) - International small and medium-sized businesses (SMBs) - North American third party (noun), third-party (adjective) video maker, video making