You just need to make sure the "auto pause" setting is turned off for whatever device you are using.  If you are using the Strava app, go to the record screen like you are starting a run.  Click on the littler gear in the upper right corner and select "auto pause".  It will show you whether it is toggled on or off for rides and runs.  Make sure it is toggled off for the run if that is what you are planning to do.

You can learn more about how Auto-Pause works in Strava from the related support article. As long as you are moving your arm (smartwatch) or bouncing around (phone), Auto-Pause won’t trigger on runs.

Of course, if preventing Auto-Pause from triggering is very important for a certain event, you can disable it entirely as @anchskier explains. Strava provides screenshots for this.