Roughly, True Strike identifies a weakness in an enemy giving you an advantage on your next attack. But it costs a full action.
Can you cast it, then use the cantrip Message as your bonus to share the info to a party member to give them the advantage dice instead of keeping it to yourself?
(This answer assumes D&D 5th edition, 2014 version, for rules and rulebook citations.)
A DM can always houserule something they find interesting and/or easier and/or clever, but by the published rules, this combination doesn't work.
Point the first: true strike doesn't work like that.
Let's look at the wording of the spell (emphasis mine)[1]:
You extend your hand and point a finger at a target in range. Your magic grants you a brief insight into the target's defenses. On your next turn, you gain advantage on your first attack roll against the target, provided that this spell hasn't ended.
The language of D&D 5e spells is particular, which matters here in two ways. Firstly, the advantage only kicks in on your next turn; not any point before the end of your next turn, on your next turn. Note that this precludes using true strike to gain advantage on an attack of opportunity, since those happen on other characters' turns.
Secondly, if the spell says "you" get an effect, you are the only person to get that effect. Contrast against the wording of guiding bolt[2]:
On a hit, the target takes 4d6 radiant damage, and the next attack roll made against this target before the end of your next turn has advantage
See the difference? Guiding bolt specifies fewer restrictions with regards to timing and who can claim the advantage than true strike. Since the wording is distinct, true strike only counts for you, and only on your next turn.
Point the second: message takes an action to cast.
Unless you have the Quickened Spell Metamagic and the sorcery points to use it, it is not possible to cast message with your bonus action anyway. There are a few routes to being able to do this (Sorcerer 3, Metamagic Adept feat), but it should be noted that the baseline action cost to use the cantrip is a standard action.
It's often overlooked, but the Help action can give advantage to an ally, with some restrictions[3]:
Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally's attack more effective. If your ally attacks the target before your next turn, the first attack roll is made with advantage.
If you wanted the flavor of using the magical insight from true strike to be the reason you are able to Help an ally in combat, using Quickened Spell to cast that cantrip as a bonus action would still leave your regular action available to use the Help action.
[1]: Player's Handbook 2014, p. 284
[2]: Player's Handbook, 2014, p. 248
[3]: Player's Handbook, 2014, p. 192
Unfortunately, the advantage you gain is magical, in some sense, and it is tied to the concentration of the spell (The concentration makes it even more costly and less valuable).
True Strike as written is really only useful in particular situations. It can be leveraged as a readied action when enemies are outside of melee range (and melee is the preferred choice), especially when combined with something that relies on advantage, like Sneak Attack. It can also be effective before combat begins, while talking to an NPC you may want to mug. But, those particular situations are rare in combat, and they don't often warrant the opportunity cost of choosing True Strike as a cantrip.
Your suggestion is creative though, and it might be worth talking about with your DM.
