Equivalent of bash’s `source` command in Powershell

To run a bash script line-by-line (so as to preserve environment variables, for instance), one does the following:

$ . myscript.sh

Or:

$ source myscript.sh

I have a PowerShell script that – among other things – sets the value of the prompt via function called prompt. Simply running this script (".\myscript.ps1") does not change the prompt. However, running function line-by-line as a command does.

So what I effectively need is something like the source command for PowerShell. Is it possible?

EDIT: Should this post be moved to Stackoverflow?

Clarification 1: Since I run this function only on a need basis (i.e., not for every powershell session and/or always), I do not want to store in my powershell startup profile.