Need help getting report filtered by a custom field with api

Hi there!
I’m having the most difficult time with something relatively simple - I’m trying to use the api to get all time entries in a specific project for entries that use a custom field called Site URL.
So for example, this api call returns all times for a project. What do I need to add to filter by customfield with id 647ddf008faddf3d23fd5de2 and TXT value?

Thanks in advance!

$headers = @{
    'content-type' = 'application/json'
    'x-api-key' = 'xxxxxxxxxxxxxxxxxxxxxxx'

$body = @{
    dateRangeStart = '2024-01-13T00:00:00.000Z'
    dateRangeEnd = '2024-03-13T23:59:59.000Z'
    summaryFilter = @{groups = @('PROJECT', 'TIMEENTRY')}
    exportType = 'JSON'
    projects = @{ids = @('xxxxxxxxxxxxxxxxx')}
} | ConvertTo-Json

$response = Invoke-RestMethod -Uri '' -Method Post -Headers $headers -Body $body