Dynamic parameter in Power Query

Date.AddDays(Date.StartOfMonth(DateTime.LocalNow()), -1) meta [IsParameterQuery=true, IsParameterQueryRequired=true]

Yes, 
see the following:

let
Today = DateTime.LocalNow(),
StartOfCurrentMonth = Date.StartOfMonth(Today),
LastMonthEnd = Date.AddDays(StartOfCurrentMonth, -1)
in
LastMonthEnd

Thank you, AlienSx  and Omid_Motamedise , for your response.

Hi 一棵黄葛树,

We sincerely appreciate your inquiry on the Microsoft Fabric Community Forum.

As per my understanding, if we intend to use the parameter, we cannot input code in the 'Current Value' field of the Manage Parameters screen; instead, we must enter the date.

As an alternative, if we wish to use the code, we need to define the parameter separately in the Advanced Editor and then reference it accordingly. Kindly refer to the attached sample code and images provided below:

let
Source = Csv.Document(File.Contents("C:\Users\v-pnaroju\Downloads\sampledata.csv"),[Delimiter=",", Columns=2, Encoding=1252, QuoteStyle=QuoteStyle.None]),
#"Promoted Headers" = Table.PromoteHeaders(Source, [PromoteAllScalars=true]),
#"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"Date", type datetime}, {"Sales", Int64.Type}}),
MyParam=Date.AddDays(Date.StartOfMonth(DateTime.LocalNow()), -1),
FilterRows=Table.SelectRows(#"Changed Type",each [Date]<MyParam)
in
FilterRows

Additionally, please find below the reference links for your convenience:
Dynamic M query parameters in Power BI Desktop - Power BI | Microsoft Learn
Parameters - Power Query | Microsoft Learn

If you find our response helpful, we kindly request you to mark it as the accepted solution and provide kudos. This will assist other community members encountering similar queries.

Thank you.

Hi 一棵黄葛树,

We have not received a response from you regarding the query and were following up to check if you have found a resolution. If you have identified a solution, we kindly request you to share it with the community, as it may be helpful to others facing a similar issue.

If you find the response helpful, please mark it as the accepted solution and provide kudos, as this will help other members with similar queries.

Thank you.