While this question has already been answered, I wanted to provide some advice for anyone still attempting this post-2022, as I was.
I'm not sure how much has changed since 2019, but all the advice I found was not very helpful at all. In the end, I found a much simpler solution - after struggling for hours and not finding anything useful.
The solution
In Power Apps, you ultimately need to convert your SelectedItems to a JSON. I set all of this on the OnChange property.
1. Create a collection from the SelectedItems result.
ClearCollect(colResult,COMBOBOX.SelectedItems)
ClearCollect(colResult,Self.SelectedItems)
2. Next, convert the collection to a JSON.
JSON(colResult)
3. [Optional] Create a variable with the result.
Set(varResult, JSON(colResult))
All of this can be in one line on the ComboBox OnChange property:
ClearCollect(colResult,Self.SelectedItems); Set(varResult,JSON(colResult))
This variable then is entered on your button that runs the Flow. The JSON will be passed to the Flow through this variable. I had about 10 combo-boxes / multiselect objects, so I had 10 variables.
YOURFLOW.Run(varResult1, varResult2, varResult3,...)
Now that you have completed the Power Apps side, you need to pick up the JSON in the Power Automate side. In Power Automate, the default setup causes issues. Below we can see two examples of this:
I believe this was setup to help, but I did not recognise this and it caused me many issues. To fix it:
4. "Switch to input entire array" (green highligh).
5. Delete everything. Select inside, press ctrl-A (to select all) and then delete.
6. In the space,
a. select Expression,
b. type "json()",
c. select between the brackets/parentheses,
d. select Dynamic content,
e. select your dynamic content, and
f. select OK.
Your full dynamic content should look something like below (since the dynamic content for myself was automatically created to be "Createitem_XXXXValue", with XXXX being the name of the SharePoint column.
json(triggerBody()['Createitem_XXXXValue'])
And that is it. No extra lines, compose or variables (initialize/set) are required anywhere.
This option is much easier than manually creating a JSON string or a collection line-by-line (all things I tried to do to brute-force a solution). I hope someone finds this and it saves them the hours of frustration and stress.
@SamEll, if you are still active, you may consider adding this to the 'accepted solutions' so others may easily find it in the future.
If I described anything incorrectly, please forgive me. I don't know much about the backend workings of Power Apps or Power Automate, or even JSONs - I'm just trying to explain what I found.
