In general there is not going to be an interaction, DMT and beta blockers primary target very different receptors. Beta blockers block adrenergic beta receptors (basically adrenaline binding sites). Whereas DMT primary binds to 5HT serotonin receptors.
The interaction would likely be a more calmer experience as adrenaline spikes won't cause the mild/ temporary increased heart rate that is associated with many psychedelics. Having said that if your beta blocker use is for a heart issue then that's a potential safety concern on its on. If its for anxiety then you should be fine or you could time your medication use and DMT use separate if appropriate to do so (if your medication use is for anxiety note if you are also taking an anti depressant then there will be an interaction with the DMT).
If you are truly worried ask the doctor for a hydophilic beta blocker like atentalol instead of a lipophilic beta blocker like propanalol. Hydophilic beta blockers won't cross the blood brain barrier readily reducing any possible interaction.
However, if you are concerned you'll cause respiratory drive issues or potential heart risks with these two drugs its extremely unlikely these two drugs would cause such issues in a healthy individual (unless you OD on beta blockers, which is generally very hard to do).
Disclaimer: All my posts are of total fiction.