As an inexorable scroller of the digital world, when you see 'HMU', it is a term for 'Hit Me Up' in text jargon. So, if they are telling you to try and contact them, it’s a method for people on platforms where messages can be sent for free online using wifi. Over your next coffee, you can say to them "Hey there!" or "Hello" just and nothing more - no fancy opening lines.
And off those words are enough for that many people who yearn for friendship! When a friend says again thinslike 'HMU when you're free', they just want to know what time suits?