From the terms used in the title of your question I understand you are creating a price alert on a ticker symbol by right-clicking on the chart and selecting "Create Alert". The default price of the alert is determined by the location you right-clicked the chart to create it.
Let's make sure everyone reading this understands this important distinction. There is another tool on Thinkorswim which is named "Study Alert". But the only way to create a Study Alert is from the MarketWatch tab. You cannot set a Study Alert from within a chart. So I am pretty sure you are describing the process of creating a simple price alert.
Since this is a price alert the time frame is completely irrelevant and the platform will display the 1 minute time frame in the preview chart. This cannot be changed. When creating a price alert, there is no point in adding chart studies to the chart, since the studies themselves are in no way connected to the fixed value you enter for the price alert. The only valid options are to select the fixed value for your alert and then select the price type and crossing direction. Chart studies can NOT be used as trigger events in the price alert tool. For this reason, Thinkorswim developers have not included any methods for adjusting the time frame of the preview chart or save any changes to the studies you add to the chart.
Now, having explained the differences between price alerts and study alerts I will include a bit more detail about Study Alerts. When you go to the MarketWatch tab and create a Study Alert, you get a preview chart which shows the alert value on the bottom subgraph. You add conditions to your Study Alert using the available tools and those conditions can be based on chart studies. The alert value displayed at the bottom shows up just like a scan signal. When the conditions you have entered are true, the alert value is 1, when the conditions you have entered are false, the alert value is 0.
Yes you can add chart studies to the preview chart when creating a Study Alert and you can also adjust the time frame. Of the two alert types I have described, this is the ONLY one in which your request is valid. (which is the only reason I took the time to describe them both). However at this time there is now way to save any of those changes. Thinkorswim developers would need to add this functionality. So if you consider this an important feature, you should submit this as a feature request to TD Ameritrade support. But please be sure to use the correct terminology and don't even bother asking for these features to be added to the price alert, because within the context of a simple price alert your request is not relevant.