Here is the snippet which helps to set the lookup field as Disable(Read-only) in the dynamics portal.
function disableLookupField(lookupId) {
$('#' + lookupId).parent().find('.input-group-btn').hide();
$('#' + lookupId).parent().parent().find('.input-group').css('width', '100%');
$('#' + lookupId + '_name').prop('disabled', true);
}
Here is the snippet which helps to Enable the lookup field in the dynamics portal.
function enableLookupField(lookupId) {
$('#' + lookupId).parent().find('.input-group-btn').show();
$('#' + lookupId + '_name').prop('disabled', false);
}