Aggregate Values from Table A into Table B

Assume I have Table A with following structure:

I also have Table B with following structure:

My objective is to aggregate all the sessions from Table A into Table B, with the following output:

As I am still very new to Power BI, I give a similar SQL expression as I would do it using this language: SUM(Sessions) FROM Table A GROUP BY channel.

Note that in the real data there are multiple different values for channel, and therefore I cannot just do a WHERE clause. Thanks!