ておく basically means to do something in advance/preparation, and 伝えておきます seems
to be a common saying for "I will tell somebody", e.g.

> 素晴らしい披露宴だったと彼に伝えておきます。

I do not understand how does ておく here work, for I see nothing about "doing in
advance" in this sentence.
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grammar set-phrases aspect
Function of ておく in 伝えておきます
I think of ておく as putting an action into place so that it する's (does its
convenient effect) later. Additionally, you get it out of your mind. You
proactively do it and get it out of your way.

Maybe there is some effect her saying will do to her husband. Something like
making him take a certain decision, putting him into a good mood so that she
can X,Y,Z...

[Here](https://www.tofugu.com/japanese-grammar/teoku/) it is descibed as "to
do something for future convenience".
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In your example, I'm guessing (needs more context!) this this would come
across as saying "I will tell him it was a wonderful reception" while implying
"...so you don't have to tell him" or "...so he doesn't have to ask".
(Depending on the context, it could imply a million other things. Ex: "...to
make him jealous for you" or "...to hide the fact that you didn't actually
have a reception")

I haven't found this particular wording, but I'd say that ておく can simply imply
that you are doing something for a positive purpose. It's similar to てしまう, but
with a positive connotation.

Another example:

> 濃厚接触者になったため、在宅勤務にしておきます。

I was exposed, so I will work from home (purpose: "to play it safe")

> 濃厚接触者になったため、在宅勤務にしてしまいまう。

I was exposed, so I will work from home (implies that the speaker has a reason
not to want to work from home)

The first example could be explained with @0149234's "for future convenience"
definition, but often times you'd use ておく subconsciously, without fully
realizing what those conveniences are--hence "for some positive purpose"
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If you said 伝えます it would most likely be understood to mean you will tell him
right away. By saying 伝えておきます it becomes clear you tell him at some point in
time before it's too late, that is, in the near future.
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