know to do还是know how to do

下面的句子出自2017年高考题的全国卷Ⅱ的七选五题目：

When people try to interrupt you, have set hours planned and let them know to come back during that time or that you’ll find them then. It’s important that you let them know when you’ll be available. It can help to eliminate future interruptions.

这里为什么用 know to come 而不用 know how to come？不是说，know 后面接不定式必须要用 how 吗？