Define your payload to send for possible more complex data

$Payload = @{ text="test string"; username="testuser" }

Use ConvertTo-Json cmdlet and Invoke-RestMethod to execute the call

Invoke-RestMethod -Uri "https://hooks.slack.com/services/yourwebhookstring" -Method Post -Body (ConvertTo-Json $Payload) 

$params = @{
    Uri = "https://your.hipchat.com/v2/room/934419/notification?auth_token=???"
    Method = "POST"
    Body = @{
        color = 'yellow'
        message = "This is a test message!"
        notify = $false 
        message_format = "text"
    } | ConvertTo-Json
    ContentType = 'application/json'
}

Invoke-RestMethod @params 

GET your REST data and store in a PowerShell object:

$Post = Invoke-RestMethod -Uri "http://jsonplaceholder.typicode.com/posts/1"

Modify your data:

$Post.title = "New Title"

PUT the REST data back

$Json = $Post | ConvertTo-Json
Invoke-RestMethod -Method Put -Uri "http://jsonplaceholder.typicode.com/posts/1" -Body $Json -ContentType 'application/json'

GET your REST data and store in a PowerShell object:

$Users = Invoke-RestMethod -Uri "http://jsonplaceholder.typicode.com/users"

Modify many items in your data:

$Users[0].name = "John Smith"
$Users[0].email = "[email protected]"
$Users[1].name = "Jane Smith"
$Users[1].email = "[email protected]"

POST all of the REST data back:

$Json = $Users | ConvertTo-Json
Invoke-RestMethod -Method Post -Uri "http://jsonplaceholder.typicode.com/users" -Body $Json -ContentType 'application/json'

Identify the item that is to be deleted and delete it:

Invoke-RestMethod -Method Delete -Uri "http://jsonplaceholder.typicode.com/posts/1"