Databound DropDownList Initial Value = a separate query value?

I have a page juggling quite a few tasks already.
The request I face is to have a value in one text box set the initial value of an "OnPageLoad" "databound" "DropDownList" from the "TextBox" value automatically.
Essentially our project numbers increment and the clients may still vary but they want the DDL to automatically select the initial client from the parent project id. While still being able to select from the list if they wish to.
I can generate the data by creating relationships from an inner join of two tables using SQL.
My dilemma is how I can tie the data from this temp table to run another search against the textbox value to actually get the required info and then tie that result to the initial value of the DDL?
Let me know if I am even making sense in describing the problem?
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