EXCEL - Set a cell's value based on a value in another worksheet

I have a spreadsheet with 2 worksheets and am trying to set a value on one worksheet based on a value in the other worksheet using an excel formula. An example follows:

Worksheet 1 (WS1) has three columns (1) WS1-ROWID, (2) WS1-EXTERNALID, (3) WS1-DESCRIPTION, and (4) WS1-QUANTITY
Worksheet 2 (WS2) has two columns (1) WS2-ROWID AND (2) WS2-EXTERNALID

There is a one to many relationship between the two worksheets. For every row in WS2 there are 1 or more rows in WS1
The worksheets are related by the WS1-EXTERNALID and the WS2-EXTERNALID

I am trying to populate WS1-ROWID in WS1 with the value found in the corresponding WS2-ROWID. So, if WS1-EXTERNALID = WS2-EXTERNALID, set WS1-ROWID = WS2-ROWID

I probably didn't explain that real well but I've also attached a sample spreadsheet that should describe what I am trying to do.

Any help will be greatly appreciated.
C--Users-cameraop-Desktop-Spreadshe.xlsx