JSFiddle - React, Tailwind, and code Playground
HTML
<input type="text" id="SearchPostcode"/>
JavaScript
$('#SearchPostcode').on('blur',function(){
var text = $('#SearchPostcode').val();
text = text.replace(/ /g,'');
var length = text.length;
var first = text.substr(0,length-3);
var last = text.slice(-3);
var final = first+' '+last;
$('#SearchPostcode').val(final);
});