Power Query Structure & Functions

Functions Used: 
Table.FromList – PowerQuery M | Microsoft Docs (Used to convert JSON data into a table format)

Splitter functions – PowerQuery M | Microsoft Docs (Identifies data is not delimited by comma or other delimeter)

let
    url = "https://191wsd17nc.execute-api.eu-west-1.amazonaws.com/production/report",
    body  = "{""reportName"":""opportunities"",
        ""reportParams"": {""groupIDs"": all}
    }",
    Parsed_JSON = Json.Document(body),
    BuildQueryString = Uri.BuildQueryString(Parsed_JSON),
    Source = Json.Document(Web.Contents(url,[
        Headers = [#"Content-Type"="application/json", #"x-api-key"="<Your Entity API Key>"],
        Content = Text.ToBinary(body) ] )),
    data = Source[data],
    #"Converted to Table" = Table.FromList(data, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Select Columns" = Table.ExpandRecordColumn(#"Converted to Table", "Column1", {"additional_info", "associatedAccounts", "close_date", "closed_status", "created", "currency", "discussions", "endUser1Region", "expected_close_date", "fiscalPeriod", "fiscalQuarter", "fiscalYear", "forecast", "grossMarginDecimal", "groupName", "id", "likely_value", "likelyValueCLF", "likelyValueCLP", "likelyValueCOP", "likelyValueUSD", "marginAmountCLF", "marginAmountCLP", "marginAmountCOP", "marginAmountUSD", "opportunitiesCRMURL", "ownerName", "poc", "productName", "projectLikelihoodGoPercentageDecimal", "proposals", "reps", "status", "title", "winGoProbabilityDecimal", "winLikelihoodGetPercentageDecimal"}, {"additional_info", "associatedAccounts", "close_date", "closed_status", "created", "currency", "discussions", "endUser1Region", "expected_close_date", "fiscalPeriod", "fiscalQuarter", "fiscalYear", "forecast", "grossMarginDecimal", "groupName", "id", "likely_value", "likelyValueCLF", "likelyValueCLP", "likelyValueCOP", "likelyValueUSD", "marginAmountCLF", "marginAmountCLP", "marginAmountCOP", "marginAmountUSD", "opportunitiesCRMURL", "ownerName", "poc", "productName", "projectLikelihoodGoPercentageDecimal", "proposals", "reps", "status", "title", "winGoProbabilityDecimal", "winLikelihoodGetPercentageDecimal"}),
    #"Renamed Columns3" = Table.RenameColumns(#"Select Columns",{{"likely_value", "OppValue"}}),
    #"Renamed Columns" = Table.RenameColumns(#"Renamed Columns3",{{"title", "Title"}, {"productName", "Product"}, {"currency", "Currency"}, {"expected_close_date", "Expected Close Date"}, {"close_date", "Actual Close Date"}, {"additional_info", "Additional Info"}, {"status", "Opportunity Status"}, {"closed_status", "Closed Status"}, {"created", "Created Date"}, {"ownerName", "Owner"}, {"forecast", "Forecast"}, {"groupName", "Group"}, {"endUser1Region", "Region"}, {"grossMarginDecimal", "GM %"}, {"projectLikelihoodGoPercentageDecimal", "Go %"}, {"winLikelihoodGetPercentageDecimal", "Get"}, {"winGoProbabilityDecimal", "Prob %"}, {"fiscalYear", "Estimated Close Date (FY)"}, {"fiscalQuarter", "Estimated Close Date (Q)"}, {"fiscalPeriod", "Estimated Close Date (FY Q)"}}),
    #"Removed Columns" = Table.RemoveColumns(#"Renamed Columns",{"opportunitiesCRMURL"}),
    #"Renamed Columns1" = Table.RenameColumns(#"Removed Columns",{{"likelyValueUSD", "Value $"}, {"marginAmountUSD", "Marg $"}, {"likelyValueCLF", "Value CLF"}, {"marginAmountCLF", "Margin CLF"}, {"likelyValueCLP", "Value CLP"}, {"marginAmountCLP", "Margin CLP"}, {"likelyValueCOP", "Value COP"}, {"marginAmountCOP", "Margin COP"}, {"id", "OppID"}}),
    #"Changed Type" = Table.TransformColumnTypes(#"Renamed Columns1",{{"Expected Close Date", type datetime}}),
    #"Renamed Columns2" = Table.RenameColumns(#"Changed Type",{{"Expected Close Date", "Excpected Close Date"}, {"reps", "_RepList"}, {"proposals", "_ProposalList"}, {"associatedAccounts", "_AssociatedAccountsList"}, {"poc", "_POCList"}, {"discussions", "_DiscussionList"}, {"Region", "State"}, {"Get", "Get %"}, {"Estimated Close Date (FY Q)", "'Estimated Close Date (FY YQ)"}, {"Estimated Close Date (Q)", "Estimated Close Date (FY Q)"}}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Renamed Columns2",{{"Margin CLP", Currency.Type}, {"Value COP", Currency.Type}, {"Margin COP", Currency.Type}, {"Value CLP", Currency.Type}, {"Margin CLF", Currency.Type}, {"Value CLF", Currency.Type},{"Marg $", Currency.Type}, {"Value $", Currency.Type}}),
    #"Renamed Columns4" = Table.RenameColumns(#"Changed Type1",{{"Excpected Close Date", "Expected Close Date"}})
in
    #"Renamed Columns4"

Group IDs

body  = "{""reportName"":""opportunities"",
        ""reportParams"": {""groupIDs"": [specific group IDs you'd like to return]}
    }",