This returns the ID (and the value) when you input your var name 
// Replace MyVar with your var name
let varName = "MyVar"
const logicVars = await Homey.logic.getVariables();
for (var iLogicVar in logicVars){
if (logicVars[iLogicVar].name == varName)
log("Variable: " + logicVars[iLogicVar].name + "\nValue:\t " + logicVars[iLogicVar].value + "\nID:\t " + logicVars[iLogicVar].id)
}
.
This is possible on 7.3.0 also @DirkG
just like Homey.logic.getVariables().then(r => log(r))
which returns all of your variables & ID’s
